MFC r362045:
Make sure packets generated by raw IP code is let through by mlx5en(4).

Allow the TCP header to reside in the mbuf following the IP header.
Else such packets will get dropped.

Backtrace:
mlx5e_sq_xmit()
mlx5e_xmit()
ether_output_frame()
ether_output()
ip_output_send()
ip_output()
rip_output()
sosend_generic()
sosend()
kern_sendit()
sendit()
sys_sendto()
amd64_syscall()
fast_syscall_common()

Sponsored by: Mellanox Technologies