tarfs: Ignore global extended headers.

Previously, we would error out if we encountered a global extended
header, because we don't know what it means. This doesn't really
matter though, and traditionally, tar implementations have either
ignored them or treated them as plain files, so just ignore them.
This allows tarfs to mount tar files created by git archive.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44600