Step 2.2:

Step 2.2:
o Shrink sglist(9) functions to work with multipage mbufs down from
  four functions to two.
o Don't use 'struct mbuf_ext_pgs *' as argument, use struct mbuf.
o Rename to something matching _epg.

Reviewed by:	gallatin
Differential Revision:	https://reviews.freebsd.org/D24598