tarfs: Silence warnings when no supported compression is enabled.

Some of the code in sys/fs/tarfs/tarfs_io.c is not specific to zstd, but is still only used when some form of decompression is enabled. Put it behind #ifdef TARFS_ZIO to silence warnings.

Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D38415