jail: Avoid multipurpose return value of function prison_ip_restrict()

Currently function prison_ip_restrict() returns true if the replacement
buffer was used, or no buffer provided and allocation fails and should
redo. The logic is confusing and cause possibly infinite loop from
eb8dcdeac22d .

Reviewed by: jamie, glebius
Approved by: kp (mentor)
Differential Revision: https://reviews.freebsd.org/D37918