sound: Safely remove channel from list in one pass.

[dev_submerge.ch]

Summary

The CHN_REMOVE_SAFE() macro did two traversals of the channel list to
remove a channel, one to check whether the channel is an element of the
list, and a second traversal through SLIST_REMOVE(). Reduce this to one
traversal, while still preventing a NULL dereference in case the channel
in question is not present in the list.

While here, rename the macro arguments to something more descriptive.

MFC after: 1 week

[dev_submerge.ch]

I stumbled upon that repeated list traversal when reviewing D48185. The implementation is roughly based on SLIST_REMOVE(). Compared to CHN_REMOVE(), the difference in performance should be only minuscule, doing one additional NULL pointer check per iteration. Therefore we may want to consider using the safe variant everywhere.

Renaming the macro arguments improves readability a lot for me, but I'm open for suggestions. I tried to take a hint or two from the SLIST_*() macros.

[christos]

LGTM. I was also about to recommend using the safe variant throughout sound(4), and just saw you mentioned it here as well. It's a good idea IMHO.