ext4: don't perform block validity checks on the journal inode

commit 0a944e8a6c66ca04c7afbaa17e22bf208a8b37f0 upstream.

Since the journal inode is already checked when we added it to the
block validity's system zone, if we check it again, we'll just trigger
a failure.

This was causing failures like this:

[   53.897001] EXT4-fs error (device sda): ext4_find_extent:909: inode
#8: comm jbd2/sda-8: pblk 121667583 bad header/extent: invalid extent entries - magic f30a, entries 8, max 340(340), depth 0(0)
[   53.931430] jbd2_journal_bmap: journal block not found at offset 49 on sda-8
[   53.938480] Aborting journal on device sda-8.

... but only if the system was under enough memory pressure that
logical->physical mapping for the journal inode gets pushed out of the
extent cache.  (This is why it wasn't noticed earlier.)

Fixes: 345c0dbf3a30 ("ext4: protect journal inode's blocks using block_validity")
Reported-by: Dan Rue <dan.rue@linaro.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
[bwh: Backported to 3.16: Use EXT4_HAS_COMPAT_FEATURE()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 02dca6c..43602f4 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -503,10 +503,15 @@
 	}
 	if (buffer_verified(bh) && !(flags & EXT4_EX_FORCE_CACHE))
 		return bh;
-	err = __ext4_ext_check(function, line, inode,
-			       ext_block_hdr(bh), depth, pblk);
-	if (err)
-		goto errout;
+	if (!EXT4_HAS_COMPAT_FEATURE(inode->i_sb,
+				     EXT4_FEATURE_COMPAT_HAS_JOURNAL) ||
+	    (inode->i_ino !=
+	     le32_to_cpu(EXT4_SB(inode->i_sb)->s_es->s_journal_inum))) {
+		err = __ext4_ext_check(function, line, inode,
+				       ext_block_hdr(bh), depth, pblk);
+		if (err)
+			goto errout;
+	}
 	set_buffer_verified(bh);
 	/*
 	 * If this is a leaf block, cache all of its entries