drivers/staging/exfat: Replace more binary semaphores with mutexes

drivers/staging/exfat: Replace more 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.
There is also lockdep support.

For both f_sem and b_sem, their semantics imply traditional lock
ownership; that is, the lock owner is the same for both lock/unlock
operations and not under irq contexts (ie for trylock/unlock scenarios).
Therefore it is safe to convert.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Link: https://lore.kernel.org/r/20191101185332.31786-1-dave@stgolabs.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>