tarfs: Fix two input validation issues.

* Reject hard or soft links with an empty target path. Currently, a
debugging kernel will hit an assertion in tarfs_lookup_path() while
a non-debugging k

tarfs: Fix two input validation issues.

* Reject hard or soft links with an empty target path. Currently, a
debugging kernel will hit an assertion in tarfs_lookup_path() while
a non-debugging kernel will happily create a link to the mount root.

* Use a temporary variable to store the result of the link target path,
and copy it to tnp->other only once we have found it to be valid.
Otherwise we error out after creating a reference to the target but
before incrementing the target's reference count, which results in a
use-after-free situation in the cleanup code.

* Correctly return ENOENT from tarfs_lookup_path() if the requested
path was not found and create_dirs is false. Luckily, existing
callers did not rely solely on the return value.

MFC after: 3 days
PR: 277360
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: sjg
Differential Revision: https://reviews.freebsd.org/D44161

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Add tarfs, a filesystem backed by tarballs.

Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: pauamma, imp
Differential Revision: https://reviews.freebsd.org/D37753