affs: Replace binary semaphores with mutexes

At a slight footprint cost (24 vs 32 bytes), mutexes are more optimal
than semaphores; it's also a nicer interface for mutual exclusion,
which is why they are encouraged over binary semaphores, when possible.

For both i_link_lock and i_ext_lock (and hence i_hash_lock which I
annotated for the hash lock mapping hackery for lockdep), their semantics
imply traditional lock ownership; that is, the lock owner is the same for
both lock/unlock operations and does not run in irq context. Therefore
it is safe to convert.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>