MFC r334844, r336180, r336458

r334844

This originated from ZFS On Linux, as
https://github.com/zfsonlinux/zfs/commit/d4a72f23863382bdf6d0ae33196f5b5decbc48fd

During scans (scrubs or resilvers), it sorts the blocks in each transaction
group by block offset; the result can be a significant improvement. (On my
test system just now, which I put some effort to introduce fragmentation into
the pool since I set it up yesterday, a scrub went from 1h2m to 33.5m with the
changes.) I've seen similar rations on production systems.

r336180

Fix up some missed and mis-merges from the sequential scan code
(r334844). Most of the changes involve moving some code around to
reduce conflicts with future merges. One of the missing changes
included a notification on scrub cancellation.

r336458

Fix a couple of typos in r334844 noticed by Richard Kojedzinszky

Approved by: mav
Sponsored by: iXsystems, Inc