tarfs: Support paths that spill into exthdrs.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44599

(cherry picked from commit b1fd95c9e24791d44593e611406b41e57826a5b8)

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

(cherry picked from commit 584e1c355ae3c994331005b7196cc87a714e5317)

tarfs: Fix 32-bit build.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D44613

(cherry picked from commit 0238d3711d9b888f678fce4274eccc9175168395)