inpcb: use locked variant of prison_check_ip*()

The pcb lookup always happens in the network epoch and in SMR section.
We can't block on a mutex due to the latter. Right now this patch opens
up a race. But soon that will be addressed by D33339.

Reviewed by: markj, jamie
Differential revision: https://reviews.freebsd.org/D33340
Fixes: de2d47842e8