subProtocol of filter didn't support custom ip protocol

[yxhl]

I want to filte sctp protocol that protocol-numbers defined. But I can't complie it successful, the golang code didn't define it and it reject filter by ip proto 132.

Another issue that i got useless bpf when i use expression imcp.

Filter expression: &{raw:icmp lexer:{reader:0xc0000f0180} buffer:{token:18 word:icmp}}
Filter: {0 2 0 16 false }
BPF instruction:
         ldh [12]
         ret #262144
         ret #0

[yxhl]

The source code define sctp as stp...

[yxhl]

Use the protocol separately for filtering will get invalid BPF

[deitch]

A PR is welcome.