btrfs: add error message for metadata level mismatch

From a recent regression report, we found that after commit 947a6299
("btrfs: move tree block parentness check into
validate_extent_buffer()") if we have a level mismatch (false alert
though), there is no error message at all.

This makes later debugging harder.  This patch will add the proper error
message for such case.

Link: https://lore.kernel.org/linux-btrfs/CABXGCsNzVxo4iq-tJSGm_kO1UggHXgq6CdcHDL=z5FL4njYXSQ@mail.gmail.com/Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -530,6 +530,9 @@ static int validate_extent_buffer(struct extent_buffer *eb,
 	}
 
 	if (found_level != check->level) {
+		btrfs_err(fs_info,
+		"level verify failed on logical %llu mirror %u wanted %u found %u",
+			  eb->start, eb->read_mirror, check->level, found_level);
 		ret = -EIO;
 		goto out;
 	}