btrfs-progs: tests: check nlinks for directories

Make sure btrfs check can detect such problem.  Right now we have no way
to fix it yet.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/fsck-tests/049-dir-hard-link/test.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+#
+# Verify that check can detect overlapped dir with 2 links
+#
+# There is a report that btrfs-check doesn't report dir with 2 links as
+# error, and only get caught by tree-checker.
+#
+# Make sure btrfs check can at least detect such error.
+
+source "$TEST_TOP/common"
+
+check_prereq btrfs
+
+check_image() {
+	run_mustfail "dir with 2 links not detected" \
+		"$TOP/btrfs" check "$1"
+}
+
+check_all_images