tarfs: Fix issues revealed by static analysis and testing.

• tarfs_alloc_mount(): Remove an unnecessary null check (CID 1504505) and an unused variable.

• tarfs_alloc_one(): Verify that the file size is not negative (CID 1504506). While there, also validate the mode, owner and group.

• tarfs_vget(), tarfs_zio_init(): Explicitly ignore return value from getnewvnode(), which cannot fail (CID 1504508)

• tarfs_lookup_path(): Fix a case where a specially-crafted tarball could trigger a null pointer dereference by first descending into, and then backing out of, a previously unknown directory. (CID 1504515)

• mktar: Construct a tarball that triggers the aforementioned null pointer dereference.

Reported by: Coverity
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: imp, kib
Differential Revision: https://reviews.freebsd.org/D38463