sbitmap: use smp_mb__after_atomic() in sbq_wake_up()

We always do an atomic clear_bit() right before we call sbq_wake_up(),
so we can use smp_mb__after_atomic(). While we're here, comment the
memory barriers in here a little more.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>