BRT: Limit brt_vdev_dump() to only one vdev

Without this patch on pool of 60 vdevs with ZFS_DEBUG enabled clone
takes much more time than copy, while heavily trashing dbgmsg for
no good reason, repeatedly dumping all vdevs BRTs again and again,
even unmodified ones.

I am generally not sure this dumping is not excessive, but decided
to keep it for now, just restricting its scope to more reasonable.

Reviewed-by: Kay Pedersen <mail@mkwg.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15625