tcp: inp_socket is valid through the lifetime of a TCP inpcb
The inp_socket is cleared only in in_pcbdetach(), which for TCP is
always accompanied with inp_pcbfree(). An inpcb that went through
in_pcbfree() shall never be returned by any kind of pcb lookup.
Reviewed by: tuexen
Differential revision: https://reviews.freebsd.org/D37062