sglist: Add sglist_append_single_mbuf().

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

Reviewed by: gallatin, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30135

(cherry picked from commit 6663f8a23e7cb60d798c5ffbd9c716b62b204f2a)