Goal of this patch is to not touch inode_lock for socket/pipes/anonfd
inodes allocation/freeing.

SINGLE dentries are attached to inodes that dont need to be linked
in a list of inodes, being "inode_in_use" or "sb->s_inodes"
As inode_lock was taken only to protect these lists, we avoid 
taking it as well.

Using iput_single() from dput_single() avoids taking inode_lock
at freeing time.

This patch has a very noticeable effect, because we avoid dirtying of 
three contended cache lines in new_inode(), and five cache lines
in iput()

(socket8 bench result : from 19.9s to 2.3s)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
 fs/anon_inodes.c   |    2 +-
 fs/dcache.c        |    2 +-
 fs/inode.c         |   29 ++++++++++++++++++++---------
 fs/pipe.c          |    2 +-
 include/linux/fs.h |   12 +++++++++++-
 net/socket.c       |    2 +-
 6 files changed, 35 insertions(+), 14 deletions(-)