btrfs-progs: tests/fsck: add test image with invalid metadata backref level

The image has a key in extent tree, (30457856 METADATA_ITEM 256), which
has invalid level (256 > BTRFS_MAX_LEVEL).

Make sure check can at least detect such problem.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/fsck-tests/053-bad-metadata-level/test.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+#
+# Verify that check can detect invalid metadata backref level.
+#
+# There is a report that btrfs-check original mode doesn't report invalid
+# metadata backref level, and lowmem mode would just crash.
+#
+# Make sure btrfs check can at least detect such error.
+
+source "$TEST_TOP/common"
+
+check_prereq btrfs
+
+check_image() {
+	run_mustfail "invalid metadata backref level not detected" \
+		"$TOP/btrfs" check "$1"
+}
+
+check_all_images