Continuation of multi page mbuf redesign from r359919.

Continuation of multi page mbuf redesign from r359919.

The following series of patches addresses three things:

Now that array of pages is embedded into mbuf, we no longer need
separate structure to pass around, so struct mbuf_ext_pgs is an
artifact of the first implementation. And struct mbuf_ext_pgs_data
is a crutch to accomodate the main idea r359919 with minimal churn.

Also, M_EXT of type EXT_PGS are just a synonym of M_NOMAP.

The namespace for the newfeature is somewhat inconsistent and
sometimes has a lengthy prefixes. In these patches we will
gradually bring the namespace to "m_epg" prefix for all mbuf
fields and most functions.

Step 1 of 4:

 o Anonymize mbuf_ext_pgs_data, embed in m_ext
 o Embed mbuf_ext_pgs
 o Start documenting all this entanglement

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