tcp: Allocate t_tcpreq_info on demand

When TCP_REQUEST_TRK is enabled, the tcb grows by 600 bytes
to accommodate the t_tcpreq_info[MAX_TCP_TRK_REQ] array.
Even when the option is enabled, not every connection is using
this feature. So let's allocate it on-demand, and save 600
bytes in the common case.

Sponsored by: Netflix
Reviewed by: rrs, tuexen
Differential Revision: https://reviews.freebsd.org/D56484