Btrfs: fix a deadlock in aborting transaction due to ENOSPC

When committing a transaction, we may bail out of running delayed refs
due to ENOSPC, and then abort the current transaction to flip into readonly.

But we'll hit a deadlock on ref head's lock since we forget to release
its lock and other cleanup stuff.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>