NFS: update comments and function definitions in fs/nfs/direct.c

Update to latest coding style standards.  Remove block comments on
statically defined functions, and place function definitions all on
one line.

Test plan:
Compile kernel with CONFIG_NFS and CONFIG_NFS_DIRECTIO.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>