inpcb: don't return INP_DROPPED entries from pcb lookups

[glebius]

Summary

The in_pcbdrop() KPI, which is used solely by TCP, allows to remove a
pcb from hash list and mark it as dropped. The comment suggests that
such pcb won't be returned by lookups. Indeed, every call to
in_pcblookup*() is accompanied by a check for INP_DROPPED. Do what
comment suggests: never return such pcbs and remove unnecessary checks.

[tuexen]

sys/netinet/tcp_lro.c

1361

Maybe also remove the comment?

[glebius]

sys/netinet/tcp_lro.c

1361

It still applies to the (tp == NULL) check. Which is going to be addressed later, and that would remove the comment, too.