jbd2: Convert jbd2_slab_create_sem to mutex

jbd2_slab_create_sem is used as a mutex, so make it one.

[ akpm muttered: We may as well make it local to
jbd2_journal_create_slab() also. ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ted Ts'o <tytso@mit.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <alpine.LFD.2.00.1010162231480.2496@localhost6.localdomain6>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>