gfs2: change from write to read lock for sd_log_flush_lock in journal replay

Function gfs2_recover_func grabs the sd_log_flush_lock rw_semaphore in
write mode. This is unnecessary because we only need to prevent log flush
from using sd_log_bio bio while it does. Therefore, a read lock will be
enough. This is a small step in cleaning up log flush.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/recovery.c

@@ -401,7 +401,7 @@ void gfs2_recover_func(struct work_struct *work)
 		/* We take the sd_log_flush_lock here primarily to prevent log
 		 * flushes and simultaneous journal replays from stomping on
 		 * each other wrt sd_log_bio. */
-		down_write(&sdp->sd_log_flush_lock);
+		down_read(&sdp->sd_log_flush_lock);
 		for (pass = 0; pass < 2; pass++) {
 			lops_before_scan(jd, &head, pass);
 			error = foreach_descriptor(jd, head.lh_tail,
@@ -412,7 +412,7 @@ void gfs2_recover_func(struct work_struct *work)
 		}
 
 		clean_journal(jd, &head);
-		up_write(&sdp->sd_log_flush_lock);
+		up_read(&sdp->sd_log_flush_lock);
 
 		gfs2_glock_dq_uninit(&thaw_gh);
 		t_rep = ktime_get();