HAMMER VFS - Fix I/O invalidation collision assertion

HAMMER VFS - Fix I/O invalidation collision assertion

Fix more issues with the hammer_reserve subsystem.

* Fix a broken base_offset calculation which had reversed the zone
  and offset fields in a HAMMER_ZONE_ENCODE() call.  The broken
  base_offset caused hammer_del_buffers() to scan the wrong offset
  range, leaving conflicting buffer cache buffers intact.

  The resulting conflict caused a KKASSERT(LIST_FIRST(&bp->b_dep) == NULL)
  to fail later on.

* Hold a reference through a potentially blocking operation in
  hammer_blockmap_alloc().

* Record the proper zone in a hammer_reserve() structure created via
  hammer_blockmap_free().  Without this the I/O invalidation code
  cannot locate conflicting hammer_buffer structures.

* hammer_reserve_setdelay_offset() was not actually placing the
  hammer_reserve() structure in the delay queue, allowing it to
  be disposed of too early.