FreeBSD: try to fallback early if can't do optimized copy

Not complete, but already shaves on some locking.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Sponsored by: Rubicon Communications, LLC ("Netgate")
Closes #14723