jbd: Split updating of journal superblock and marking journal empty

There are three case of updating journal superblock. In the first case, we want
to mark journal as empty (setting s_sequence to 0), in the second case we want
to update log tail, in the third case we want to update s_errno. Split these
cases into separate functions. It makes the code slightly more straightforward
and later patches will make the distinction even more important.

Signed-off-by: Jan Kara <jack@suse.cz>

fs/jbd/commit.c

@@ -308,7 +308,7 @@ void journal_commit_transaction(journal_t *journal)
 	/* Do we need to erase the effects of a prior journal_flush? */
 	if (journal->j_flags & JFS_FLUSHED) {
 		jbd_debug(3, "super block updated\n");
-		journal_update_superblock(journal, 1);
+		journal_update_sb_log_tail(journal);
 	} else {
 		jbd_debug(3, "superblock not updated\n");
 	}