tcp_timers: check for (INP_TIMEWAIT | INP_DROPPED) only once

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.

Reviewed by: rrs, hselasky
Differential revision: https://reviews.freebsd.org/D32967