Honor net.inet.tcp.drop_synfin in other states than LISTEN

[tuexen]

Summary

Currently the sysctl variable net.inet.tcp.drop_synfin is not honored in all states, for example not in SYN-SENT.
This patch adds code to check the sysctl variable in other states than LISTEN.

Test Plan

Get all tests from
https://github.com/freebsd-net/tcp-testsuite/blob/master/state-event-engine/rcv-syn-fin-syn-sent/README.md
passed

[gnn]

Other than the need for some clarifying parentheses I think this is OK.

sys/netinet/tcp_input.c

1557

I'd like some clarifying parentheses here as well as in all the other similar if statements below.

[tuexen]

Address gnn's comment by adding parentheses.

[ae]

sys/netinet/tcp_input.c

248

This declaration seems unnecessary. It is already defined at the line 147.

[tuexen]

Address Andrey's comment.