NFS: if ATTR_KILL_S*ID bits are set, then skip mode change

If the ATTR_KILL_S*ID bits are set then any mode change is only for clearing
the setuid/setgid bits.  For NFS, skip the mode change and let the server
handle it.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>