[PATCH] knfsd: improve the test for cross-device-rename in nfsd

Just testing the i_sb isn't really enough, at least the vfsmnt must be the
same.  Thanks Al.

Cc: Al Viro <viro@ftp.linux.org.uk>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>