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.

Since v2 this series has been reworked so that the iunlink code is more
robust in the face of backref cache failures (since we can always fall
back to the old bucket walking code), removal of the unlinked inode
counts, standardization of the helper function parameter types, and
fixing other review comments.