BRT: Relax brt_pending_apply() locking

Since brt_pending_apply() is running in syncing context, no other
brt_pending_tree accesses are possible for the TXG. We don't need
to acquire brt_pending_lock here.

Reviewed-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15955