affs: convert s_bmlock into a mutex

The semaphore s_bmlock is used as a mutex.  Convert it to the mutex API.

Signed-off-by: Matthias Kaehlcke <matthias@kaehlcke.net>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>