xfs: incore unlinked list

This new patch series refactors the existing code that handles metadata
updates to the unlinked list when adding or removing inodes from that
list.  It then adds an in-core hashtable to record which inode's
next_unlinked field points to a given inode.  This enables us to remove
any inode from the on-disk unlinked structure without having to actually
walk the entire unlinked list, which reduces overhead substantially.