reiserfs: convert j_lock to mutex

j_lock is a semaphore but uses it as if it were a mutex.  This patch converts
it to a mutex.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Edward Shishkin <edward.shishkin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>