jail: Make prison_owns_vnet() operate on a prison instead of a ucred

This will be useful in an upcoming change. No functional change
intended.

Reviewed by: jamie
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51524

(cherry picked from commit 748a4ea1caffca48c4949d5a7b964853c44fbdae)