jbd2: make jbd2_handle_buffer_credits() handle reserved handles

The helper jbd2_handle_buffer_credits() doesn't correctly handle reserved
handles which can lead to crashes. Fix it getting of journal pointer to
work for reserved handles as well.

Fixes: a9a8344 ("ext4, jbd2: Provide accessor function for handle credits")
Reported-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191115102210.29445-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

include/linux/jbd2.h

@@ -1627,10 +1627,14 @@ static inline tid_t  jbd2_get_latest_transaction(journal_t *journal)
 	return tid;
 }
 
-
 static inline int jbd2_handle_buffer_credits(handle_t *handle)
 {
-	journal_t *journal = handle->h_transaction->t_journal;
+	journal_t *journal;
+
+	if (!handle->h_reserved)
+		journal = handle->h_transaction->t_journal;
+	else
+		journal = handle->h_journal;
 
 	return handle->h_total_credits -
 		DIV_ROUND_UP(handle->h_revoke_credits_requested,