CIFS: ignore mode change if it's just for clearing setuid/setgid bits

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

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Steven French <sfrench@us.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>