Group per-domain reservation data in the same structure.

We currently have the per-domain partially populated reservation queues
and the per-domain queue locks. Define a new per-domain padded
structure to contain both of them. This puts the queue fields and lock
in the same cache line and avoids the false sharing within the old queue
array.

Also fix field packing in the reservation structure. In many places we
assume that a domain index fits in 8 bits, so we can do the same there
as well. This reduces the size of the structure by 8 bytes.

Update some comments while here. No functional change intended.

Reviewed by: dougm, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22391