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