Add two functions that create M_EXTPG mbufs with anonymous pages.

These two functions are needed by nfs-over-tls, but could also be
useful for other purposes.
mb_alloc_ext_plus_pages() - Allocates a M_EXTPG mbuf and enough anonymous

pages to store "len" data bytes.

mb_mapped_to_unmapped() - Copies the data from a list of mapped (non-M_EXTPG)

mbufs into a list of M_EXTPG mbufs allocated with anonymous pages.
This is roughly the inverse of mb_unmapped_to_ext().

Reviewed by: gallatin
Differential Revision: https://reviews.freebsd.org/D25182