May be related to #528

@antoninbas please check

The API changes look good to me, but I am not a maintainer for this repo. Hopefully users don't find it confusing that Rule{Mark: 0} means "match all marks" but Rule{Mark:1} means "match mark 1 exclusively". To match mark 0 exclusively, one needs to use Rule{Mark: 0, Mask: &mask} with mask := uint32(0xffffffff). If other believe that this is confusing, the only solution is to make Mark a pointer as well, which also impacts usability to some extent.

~@ubuntu:~$ ip rule add fwmark 0 table 300 pref 120
~@ubuntu:~$ ip rule add fwmark 1 table 300 pref 120
~@ubuntu:~$ ip rule add fwmark 0/0xFFFFFFFF table 300 pref 130
~@ubuntu:~$ ip rule add fwmark 1/0xFFFFFFFF table 300 pref 130
~@ubuntu:~$ ip rule show
120:	from all lookup 300  <-------  mark 0 match all marks
120:	from all fwmark 0x1 lookup 300
130:	from all fwmark 0 lookup 300
130:	from all fwmark 0x1 lookup 300

I used the ip command to add rules with fwmark:0, fwmark:1, fwmark:0/0xFFFFFFFF, and fwmark: 1/0xFFFFFFFF. There's a similar confusing phenomenon here.
As an API interface, should we replicate the behavior of the ip command, or should we avoid potentially confusing behavior internally in the API.
What should be the decision?
@antoninbas

@qxoqx I was unaware that there was a similar behavior with ip rule, thanks for pointing it out.
Then I would say the current version looks good to me, but again I am not a maintainer for this repo so the decision is not mine.

Thanks @antoninbas

LGTM

Thank you all for review the PR!