tcp: improve consistency of SYN-cache handling

[tuexen]

Summary

Originally, a SYN-cache entry was always allocated and later freed, when not needed anymore. Then the allocation was avoided, when no SYN-cache entry was needed, and a copy on the stack was used. But the logic regarding freeing was not updated.
This patch doesn't re-check conditions (which may have changed) when deciding to insert or free the entry, but uses the result of the earlier check. This simplifies the code and improves also consistency.