gh-149486: tarfile.data_filter: validate written link target (GH-149487)

gh-149486: tarfile.data_filter: validate written link target (GH-149487)

The data filter rewrote linknames with normpath() but ran the
containment check against the un-normalised value, and computed a
symlink's directory before stripping trailing slashes.  Both let a
crafted archive create links pointing outside the destination.  Also
reject link members that resolve to the destination directory itself,
which could otherwise replace it with a symlink and redirect all
subsequent members.

(Patch by Greg; Petr's just reviewing & merging.)

Co-authored-by: Gregory P. Smith <greg@krypto.org>