knfsd: only set ATTR_KILL_S*ID if ATTR_MODE isn't being explicitly set

It's theoretically possible for a single SETATTR call to come in that sets the
mode and the uid/gid.  In that case, don't set the ATTR_KILL_S*ID bits since
that would trip the BUG() in notify_change.  Just fix up the mode to have the
same effect.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: Neil Brown <neilb@suse.de>
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>