brt_vdev_realloc: use vmem_alloc for large allocation

bv_entcount can be a relatively large allocation (see comment for
BRT_RANGESIZE), so get it from the big allocator.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kay Pedersen <mail@mkwg.de>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-By: OpenDrives Inc.
Sponsored-By: Klara Inc.
Closes #15050