BRT: More optimizations after per-vdev splitting

• With both pending and current AVL-trees being per-vdev and having

effectively identical comparison functions (pending tree compared
also birth time, but I don't believe it is possible for them to be
different for the same offset within one transaction group), it
makes no sense to move entries from one to another. Instead inline
dramatically simplified brt_entry_addref() into brt_pending_apply().
It no longer requires bv_lock, since there is nothing concurrent
to it at the time. And it does not need to search the tree for the
previous entries, since it is the same tree, we already have the
entry and we know it is unique.

• Put brt_vdev_lookup() and brt_vdev_addref() into different tree

traversals to avoid false positives in the first due to the second
entcount modifications. It saves dramatic amount of time when a
file cloned first time by not looking for non-existent ZAP entries.

• Remove avl_is_empty(bv_tree) check from brt_maybe_exists(). I

don't think it is needed, since by the time all added entries are
already accounted in bv_entcount. The extra check must be producing
too many false positives for no reason. Also we don't need bv_lock
there, since bv_entcount pointer must be table at this point, and
we don't care about false positive races here, while false negative
should be impossible, since all brt_vdev_addref() have already
completed by this point. This dramatically reduces lock contention
on massive deletes of cloned blocks. The only remaining one is
between multiple parallel free threads calling brt_entry_decref().

• Do not update ZAP if net change for a block over the TXG was 0.

In combination with above it makes file move between datasets as
cheap operation as originally intended if it fits into one TXG.

• Do not allocate vdevs on pool creation or import if it did not

have active block cloning. This allows to save a bit in few cases.

• While here, add proper error handling in brt_load() on pool

import instead of assertions.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16773