vfs: fix shrink_submounts

In the last refactoring of shrink_submounts a variable was not completely
renamed.  So finish the renaming of mnt to m now.

Without this if you attempt to mount an nfs mount that has both automatic
nfs sub mounts on it, and has normal mounts on it.  The unmount will
succeed when it should not.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>