r/o bind mounts: give permission() a local 'mnt' variable

r/o bind mounts: give permission() a local 'mnt' variable

First of all, this makes the structure jumping look a little bit cleaner.  So,
this stands alone as a tiny cleanup.  But, we also need 'mnt' by itself a few
more times later in this series, so this isn't _just_ a cleanup.

Signed-off-by: Dave Hansen <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>