jbd2: minimize region locked by j_list_lock in journal_get_create_access()

It's not needed until we start trying to modifying fields in the
journal_head which are protected by j_list_lock.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/transaction.c

@@ -1073,7 +1073,6 @@ int jbd2_journal_get_create_access(handle_t *handle, struct buffer_head *bh)
 	 * reused here.
 	 */
 	jbd_lock_bh_state(bh);
-	spin_lock(&journal->j_list_lock);
 	J_ASSERT_JH(jh, (jh->b_transaction == transaction ||
 		jh->b_transaction == NULL ||
 		(jh->b_transaction == journal->j_committing_transaction &&
@@ -1096,12 +1095,14 @@ int jbd2_journal_get_create_access(handle_t *handle, struct buffer_head *bh)
 		jh->b_modified = 0;
 
 		JBUFFER_TRACE(jh, "file as BJ_Reserved");
+		spin_lock(&journal->j_list_lock);
 		__jbd2_journal_file_buffer(jh, transaction, BJ_Reserved);
 	} else if (jh->b_transaction == journal->j_committing_transaction) {
 		/* first access by this transaction */
 		jh->b_modified = 0;
 
 		JBUFFER_TRACE(jh, "set next transaction");
+		spin_lock(&journal->j_list_lock);
 		jh->b_next_transaction = transaction;
 	}
 	spin_unlock(&journal->j_list_lock);