staging: comedi: COMEDI_BUFINFO: some behavioural changes

[Ian Abbott <abbotti@mev.co.uk>]

These patches change the behavior of the `COMEDI_BUFINFO` ioctl, which
is used to manage buffer positions for a previously set up asynchronous
acquisition command.  It is used instead of the read and write file
operations when the buffer has been mmapped.

Patches 1 to 4 are fairly innocuous.  Patches 5 to 8 change the error
handling when the asynchronous command is no longer running, allow the
subdevice to become "non-busy" automatically in more cases (causing
subsequent calls to return `-EINVAL`), and report abnormal command
termination with return value `-EPIPE` (which also makes the subdevice
become non-busy).

1) staging: comedi: COMEDI_BUFINFO: get amount freed, not amount
   allocated
2) staging: comedi: COMEDI_BUFINFO: force bytes_read or bytes_written to
   0
3) staging: comedi: COMEDI_BUFINFO: update buffer before becoming
   non-busy
4) staging: comedi: COMEDI_BUFINFO: force bytes_written to 0 if stopped
5) staging: comedi: COMEDI_BUFINFO: return error if no active command
6) staging: comedi: COMEDI_BUFINFO: become non-busy even if bytes_read
   is 0
7) staging: comedi: COMEDI_BUFINFO: return -EPIPE for abnormal read
8) staging: comedi: COMEDI_BUFINFO: terminate "write" command when
   stopped

 drivers/staging/comedi/comedi_fops.c | 84 +++++++++++++++++++-----------------
 1 file changed, 44 insertions(+), 40 deletions(-)