implement zfs_vptocnp() using z_parent property

This should allow vn_fullpath() to work even when vfs name cache is
disabled for zfs, which is the case when zfs properties like
casesensitivity and normalization are set non-default values.

The new code should be 100% reliable for directories and "mostly"
reliable for files, that is, when hardlinks across directories are
not used.

Reported by: Frederic Chardon <chardon.frederic@gmail.com>
Reviewed by: kib (vfs contract)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D8146