MFV 316871

7490 real checksum errors are silenced when zinject is on

illumos/illumos-gate@6cedfc397d92d64e442f0aae4445ac507beaf58f
https://github.com/illumos/illumos-gate/commit/6cedfc397d92d64e442f0aae4445ac507beaf58f

https://www.illumos.org/issues/7490

When zinject is on, error codes from zfs_checksum_error() can be overwritten
due to an incorrect and overly-complex if condition.

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Pavel Zakharov <pavel.zakharov@delphix.com>