Map ECKSUM and EFRAGS from ZFS onto real errnos.

Make it less confusing when, for example, stat sets errno to 122 because a
checksum failed in ZFS:

Before: getfacl: /foo/bar: stat() failed: Unknown error: 122
After: getfacl: /foo/bar: stat() failed: Integrity check failed

Submitted by: Ryan Moeller <ryan@ixsystems.com>
Reviewed by: mckusick, mav
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D22973