tcp: close two minor races with debug messages

The syncache entry is locked by the hash bucket lock. After running
SCH_UNLOCK(), we have no guarantee that the syncache entry still
exists.

Resolve the race by moving SCH_UNLOCK() after the log() call which
reads variables from the syncache entry.

Reviewed by: rrs, tuexen, Nick Banks
Sponsored by: Netflix
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52868