tcp: fix sending RST after second inp lookup

When we first find an inp, we set also the tp. If then a second
lookup is necessary, the inp is recomputed. If this fails, the
tp is not cleared, which resulted in failing KASSERT.
Therefore, clear the tp when staring the inp lookup procedure.

Reported by: Jenkins
Fixes: 02d15215cef2 ("tcp: improve blackhole support")
Sponsored by: Netflix, Inc.

(cherry picked from commit df9de82f54d13aa077889349e15d9bf7794a1e6f)