tcp: purge the input queue from tcp_discardcb()

The purge was intentionally removed in a540cdca3183. My assumption
was that the stacks that use the input queue always call the
tcp_handle_orphaned_packets() in their tfb_tcp_fb_fini method.
However, rack will skip doing that if t_fb_ptr is NULL and there are
scenarios when it is NULL, e.g. close(2) on a socket (but some
special close(2)). Instead of working out all possible scenarios
let's put this safebelt back.

Reviewed by: rrs
Differential Revision: https://reviews.freebsd.org/D39696