memdesc: Add a MEMDESC_VMPAGES descriptor type.

This memory descriptor is backed by an array of VM pages. This type
requires adding a new field to 'struct memdesc' to hold the offset
within the first page. For LP64 systems, this new field is added in
an existing padding hole so does not increase the size. For ILP32
systems, this grows 'struct memdesc' from 12 to 16 bytes.

Reviewed by: imp, markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D41028