sglist: Add sglist_append_single_mbuf().

[jhb]

Summary

This function appends the contents of a single mbuf to an sglist
rather than an entire mbuf chain.

Sponsored by: Netflix

[jhb]

I kind of wish "sglist_append_mbuf" was the "single" version and there was a separate name for the "append a mbuf chain" variant, but that ship has sailed I think.

[markj]

In D30135#676970, @jhb wrote:

I kind of wish "sglist_append_mbuf" was the "single" version and there was a separate name for the "append a mbuf chain" variant, but that ship has sailed I think.

Yeah I had the same thought while reading these diffs.