jbd2: make jbd2_handle_buffer_credits() handle reserved handles

mainline inclusion
from mainline-5.5-rc1
commit 3c845acd0237caef617f330a0e3b37ad8ae9fea5
category: bugfix
bugzilla: 25031
CVE: NA
---------------------------

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: a9a8344ee171 ("ext4, jbd2: Provide accessor function for handle credits")
Reported-by: NEric Biggers <ebiggers@kernel.org>
Signed-off-by: NJan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20191115102210.29445-1-jack@suse.czSigned-off-by: NTheodore Ts'o <tytso@mit.edu>
Signed-off-by: Nzhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: NYang Erkun <yangerkun@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

include/linux/jbd2.h

@@ -1654,10 +1654,14 @@ 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,