ext4: fix block validity checks for journal inodes using indirect blocks

commit 170417c8c7bb2cbbdd949bf5c443c0c8f24a203b upstream.

Commit 345c0dbf3a30 ("ext4: protect journal inode's blocks using
block_validity") failed to add an exception for the journal inode in
ext4_check_blockref(), which is the function used by ext4_get_branch()
for indirect blocks.  This caused attempts to read from the ext3-style
journals to fail with:

[  848.968550] EXT4-fs error (device sdb7): ext4_get_branch:171: inode #8: block 30343695: comm jbd2/sdb7-8: invalid block

Fix this by adding the missing exception check.

Fixes: 345c0dbf3a30 ("ext4: protect journal inode's blocks using block_validity")
Reported-by: Arthur Marsh <arthur.marsh@internode.on.net>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.16: Use EXT4_HAS_COMPAT_FEATURE]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
index f4f8059..2a7efb6 100644
--- a/fs/ext4/block_validity.c
+++ b/fs/ext4/block_validity.c
@@ -277,6 +277,12 @@
 	__le32 *bref = p;
 	unsigned int blk;
+	if (EXT4_HAS_COMPAT_FEATURE(inode->i_sb,
+	    (inode->i_ino ==
+	     le32_to_cpu(EXT4_SB(inode->i_sb)->s_es->s_journal_inum)))
+		return 0;
 	while (bref < p+max) {
 		blk = le32_to_cpu(*bref++);
 		if (blk &&