tarfs: Fix two input validation issues.

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