gfs2: Move lock flush locking to gfs2_trans_{begin,end}

Move the read locking of sd_log_flush_lock from gfs2_log_reserve to
gfs2_trans_begin, and its unlocking from gfs2_log_release to
gfs2_trans_end.  Use gfs2_log_release in two places in which it was open
coded before.

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