Fix ASSERT in dsl_scan_fini() and cleanup comments

Fix ASSERT in dsl_scan_fini() and cleanup comments

This patch fixes an issue where dsl_scan_prefetch_cb() might
add more prefetch I/Os to the prefetch queue after prefetching
has been completed. This was happening because that code was
checking scn->scn_suspending instead of scn->scn_prefetch_stop.
This occasionally triggered an ASSERT during ztest runs in
dsl_scan_fini() when the code attempted to destroy an AVL tree
that still had entires in it. This patch also includes a number
of spelling corrections and comment cleanups throughout
dsl_scan.c

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7353