gfs2: Clean up gfs2_log_reserve

Wake up log waiters in gfs2_log_release when log space has actually become
available.  This is a much better place for the wakeup than gfs2_logd.

Check if enough log space is immeditely available before anything else.  If
there isn't, use io_wait_event to wait instead of open-coding it.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>