[BUG]
When running fuzz-tests/003 and fuzz-tests/009, btrfs-progs will crash
due to BUG_ON().

[CAUSE]
We abused BUG_ON() in btrfs_commit_transaction(), which is one of the
most error prone function for fuzzed images.

Currently to cleanup the aborted transaction, we only need to clean up
the only per-transaction data: delayed refs.

This patch will introduce a new function, btrfs_destroy_delayed_refs()
to cleanup delayed refs when we failed to commit transaction.

With that function, we will gently destroy per-trans delayed ref, and
remove the BUG_ON()s in btrfs_commit_transaction().

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