BRT: Rework structures and locks to be per-vdev

While block cloning operation from the beginning was made per-vdev,
before this change most of its data were protected by two pool-
wide locks. It created lots of lock contention in many workload.

This change makes most of block cloning data structures per-vdev,
which allows to lock them separately. The only pool-wide lock now
it spa_brt_lock, protecting array of per-vdev pointers and in most
cases taken as reader. Also this splits per-vdev locks into three
different ones: bv_pending_lock protects the AVL-tree of pending
operations in open context, bv_mos_entries_lock protects BRT ZAP
object from while being prefetched, and bv_lock protects the rest
of per-vdev context during TXG commit process. There should be
no functional difference aside of some optimizations.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pawel Jakub Dawidek <pjd@FreeBSD.org>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16740