[PATCH net] dpll: fix device-id-get and pin-id-get to return errors properly

[Petr Oros <poros@redhat.com>]

The device-id-get and pin-id-get handlers were ignoring errors from
the find functions and sending empty replies instead of returning
error codes to userspace.

When dpll_device_find_from_nlattr() or dpll_pin_find_from_nlattr()
returned an error (e.g., -EINVAL for "multiple matches" or -ENODEV
for "not found"), the handlers checked `if (!IS_ERR(ptr))` and
skipped adding the device/pin handle to the message, but then still
sent the empty message as a successful reply.

This caused userspace tools to receive empty responses with id=0
instead of proper netlink errors with extack messages like
"multiple matches".

The bug is visible via strace, which shows the kernel sending TWO
netlink messages in response to a single request:

1. Empty reply (20 bytes, just header, no attributes):
   recvfrom(3, [{nlmsg_len=20, nlmsg_type=dpll, nlmsg_flags=0, ...},
                {cmd=0x7, version=1}], ...)

2. NLMSG_ERROR ACK with extack (because of NLM_F_ACK flag):
   recvfrom(3, [{nlmsg_len=60, nlmsg_type=NLMSG_ERROR,
                 nlmsg_flags=NLM_F_CAPPED|NLM_F_ACK_TLVS, ...},
                [{error=0, msg={...}},
                 [{nla_type=NLMSGERR_ATTR_MSG}, "multiple matches"]]], ...)

The C YNL library parses the first message, sees an empty response,
and creates a result object with calloc() which zero-initializes all
fields, resulting in id=0.

The Python YNL library parses both messages and displays the extack
from the second NLMSG_ERROR message.

Fix by checking `if (IS_ERR(ptr))` first and returning the error
code immediately, so that netlink properly sends only NLMSG_ERROR with
the extack message to userspace. After this fix, both C and Python
YNL tools receive only the NLMSG_ERROR and behave consistently.

This affects:
- DPLL_CMD_DEVICE_ID_GET: now properly returns error when multiple
  devices match the criteria (e.g., same module-name + clock-id)
- DPLL_CMD_PIN_ID_GET: now properly returns error when multiple pins
  match the criteria (e.g., same module-name)

Before fix:
  $ dpll pin id-get module-name ice
  0  (wrong - should be error, there are 17 pins with module-name "ice")

After fix:
  $ dpll pin id-get module-name ice
  Error: multiple matches
  (correct - kernel reports the ambiguity via extack)

Signed-off-by: Petr Oros <poros@redhat.com>
---
 drivers/dpll/dpll_netlink.c | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/drivers/dpll/dpll_netlink.c b/drivers/dpll/dpll_netlink.c
index 74c1f0ca95f24a..a4153bcb6dcfe1 100644
--- a/drivers/dpll/dpll_netlink.c
+++ b/drivers/dpll/dpll_netlink.c
@@ -1559,16 +1559,18 @@ int dpll_nl_pin_id_get_doit(struct sk_buff *skb, struct genl_info *info)
 		return -EMSGSIZE;
 	}
 	pin = dpll_pin_find_from_nlattr(info);
-	if (!IS_ERR(pin)) {
-		if (!dpll_pin_available(pin)) {
-			nlmsg_free(msg);
-			return -ENODEV;
-		}
-		ret = dpll_msg_add_pin_handle(msg, pin);
-		if (ret) {
-			nlmsg_free(msg);
-			return ret;
-		}
+	if (IS_ERR(pin)) {
+		nlmsg_free(msg);
+		return PTR_ERR(pin);
+	}
+	if (!dpll_pin_available(pin)) {
+		nlmsg_free(msg);
+		return -ENODEV;
+	}
+	ret = dpll_msg_add_pin_handle(msg, pin);
+	if (ret) {
+		nlmsg_free(msg);
+		return ret;
 	}
 	genlmsg_end(msg, hdr);
 
@@ -1735,12 +1737,14 @@ int dpll_nl_device_id_get_doit(struct sk_buff *skb, struct genl_info *info)
 	}
 
 	dpll = dpll_device_find_from_nlattr(info);
-	if (!IS_ERR(dpll)) {
-		ret = dpll_msg_add_dev_handle(msg, dpll);
-		if (ret) {
-			nlmsg_free(msg);
-			return ret;
-		}
+	if (IS_ERR(dpll)) {
+		nlmsg_free(msg);
+		return PTR_ERR(dpll);
+	}
+	ret = dpll_msg_add_dev_handle(msg, dpll);
+	if (ret) {
+		nlmsg_free(msg);
+		return ret;
 	}
 	genlmsg_end(msg, hdr);
 
-- 
2.51.0