ipdivert: check that PCB is still valid after taking INPCB_RLOCK.

We are inspecting PCBs of divert sockets under NET_EPOCH section,
but PCB could be already detached and we should check INP_FREED flag
when we took INP_RLOCK.

PR: 254478
Differential Revision: https://reviews.freebsd.org/D29420

(cherry picked from commit c80a4b76ceacc5aab322e7ac1407eea8c90cb3b1)