NFSD: use explicit lock/unlock for directory ops

When creating or unlinking a name in a directory use explicit
inode_lock_nested() instead of fh_lock(), and explicit calls to
fh_fill_pre_attrs() and fh_fill_post_attrs().  This is already done
for renames, with lock_rename() as the explicit locking.

Also move the 'fill' calls closer to the operation that might change the
attributes.  This way they are avoided on some error paths.

For the v2-only code in nfsproc.c, the fill calls are not replaced as
they aren't needed.

Making the locking explicit will simplify proposed future changes to
locking for directories.  It also makes it easily visible exactly where
pre/post attributes are used - not all callers of fh_lock() actually
need the pre/post attributes.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>