Fix the signature for zone_import and zone_release

These are cast to uma_import and uma_release functions. Use the signature
for these in the zone functions.

This was found with an experimental Kernel CFI. It will complain if the
signature is different than what a function pointer expects. The
simplest way to fix these is to correct the signature.

Reviewed by: rlibby
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22671