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

(cherry picked from commit 38b3683592d4c20a74f52a6e8e29368e6fa61858)

tarfs: Improve validation of numeric fields.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: sjg, allanjude
Differential Revision: https://reviews.freebsd.org/D44166

(cherry picked from commit 8427d94ce05682abb6c75e2a27c8c497962c0dc5)

tarfs: Avoid overflow in exthdr calculation.

MFC after: 3 days
PR: 277420
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D44202

(cherry picked from commit c291b7914e1db9469cc820abcb1f5dde7a6f7f28)

tarfs: Remove unnecessary hack and obsolete comment.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44203

(cherry picked from commit e212f0c0666e7d3a24dce03b8c88920d14b80e47)

tarfs: Fix checksum calculation.

The checksum code assumed that struct ustar_header filled an entire
block and calculcated the checksum based on the size of the structure.
The header is in fact only 500 bytes long while the checksum covers
the entire block (“logical record” in POSIX terms). Add padding and
an assertion, and clean up the checksum code.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44226

(cherry picked from commit 0118b0c8e58a438a931a5ce1bf8d7ae6208cc61b)

tarfs: Factor out common test code.

MFC after: 3 days
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44227

(cherry picked from commit 32b8aac6f9b77a1c4326083472d634e5de427547)

tarfs: Fix checksum on 32-bit platforms.

MFC after: 3 days
Fixes: b56872332e47786afc09515a4daaf1388da4d73c
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D44261

(cherry picked from commit cbddb2f02c7687d1039abcffd931e94e481c11a5)