Improve sorted scan memory accounting

Since we use two B-trees q_exts_by_size and q_exts_by_addr, we should
count 2x sizeof (range_seg_gap_t) per node. And since average B-tree
memory efficiency is about 75%, we should increase it to 3x.

Previous code under-counted up to 30% of the memory usage.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #13537