Dump is hard-wired to believe that it can read disks on
1024-byte boundaries. For many years this was a reasonable
assumption. However, in recent years we have begun seeing
devices with 2048-byte sectors. These devices return errors
when dump tries to read starting in the middle of a sector
or when it tries to read only the first half of a sector.
Rather than change the native block size used by dump (and
thus create an incompatible dump format), this fix checks
for transfer requests that start and/or end on a non-sector
boundary. When such a read is detected, the new code reads
the entire sector and copies out just the part that dump
needs.

Reviewed by: Poul-Henning Kamp <phk@critter.freebsd.dk>
Approved by: re (John Baldwin <jhb@FreeBSD.org>)
Sponsored by: DARPA & NAI Labs.