zisofs use mutex instead of semaphore

Use mutex instead of semaphore in fs/isofs/compress.c, and remove an
unnecessary variable.

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>