jail: Use flexible array member within struct prison_ip

Current implementation utilize off-by-one struct prison_ip to access the
IPv[46] addresses. It is error prone and hence comes the regression fix
21ad3e27fabc and ddbf879d79d4. Use flexible array member so that compiler
will catch such errors and it will also be easier to review.

No functional change intended.

Reviewed by: melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D37874