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: NNeil Brown <neilb@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>