hammer2 - Remove the hidden directory, rework deletions
* Now that inodes are separately indexed we no longer need the hidden
directory abstraction to handle unlinked-but-open files. Get rid of
ALL the hidden directory handling code.
* Rework xop_unlink and hammer2_inode_unlink_finisher(). We cannot safely
reference the inode chain's inode data to get the nlinks count. Instead,
figure it all out on the frontend using the active nlinks in the
hammer2_inode_t structure.
* Fixes hardlink removal and rename issues.