tcp_timers: check for (INP_TIMEWAIT | INP_DROPPED) only once

[glebius]

Summary

All timers keep inpcb locked through their execution. We need to
check these flags only once. Checking for INP_TIMEWAIT earlier is
is also safer, since such inpcbs point into tcptw rather than tcpcb,
and any dereferences of inp_ppcb as tcpcb are erroneous.

[• hselasky]

This check is occurring lots of places and could have been factored out:

if (inp->inp_flags & (INP_TIMEWAIT | INP_DROPPED)) {
        INP_WUNLOCK(inp);