pf: Use a macro to get the hash row in pf_find_state_byid()

[markj]

Summary

This seems a bit preferable to open-coding it. No functional change
intended.

[glebius]

I personally would prefer something without underscore prefix. The latter usually means something internal and not supposed to be used. But both macro working on ID and macro working on state definitely have the same level of visibility and usage - are supposed to be used only in pf(4) internally.

I can suggest two options:

PF_IDHASHID(id)
PF_IDHASH(state)

or

PF_IDHASH(id)
PF_STHASH(state)

More options are also possible.

[markj]

Use a different name for the ID hash macro.

[glebius]

https://reviews.freebsd.org/D49518