toe: Read-lock the inp in toe_4tuple_check().
tcp_twcheck now expects a read lock on the inp for the SYN case
instead of a write lock.
Reviewed by: np
Fixes: 1db08fbe3ffa tcp_input: always request read-locking of PCB for any pure SYN segment.
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D30782