Add a new populate() pager method and extend device pager ops vector
with cdev_pg_populate() to provide device drivers access to it. It
gives drivers fine control of the pages ownership and allows drivers
to implement arbitrary prefault policies.

The populate method is called on a page fault and is supposed to
populate the vm object with the page at the fault location and some
amount of pages around it, at pager's discretion. VM provides the
pager with the hints about current range of the object mapping, to
avoid instantiation of immediately unused pages, if pager decides so.
Also, VM passes the fault type and map entry protection to the pager,
allowing it to force the optimal required ownership of the mapped
pages.

Installed pages must contiguously fill the returned region, be fully
valid and exclusively busied. Of course, the pages must be compatible
with the object' type.

After populate() successfully returned, VM fault handler installs as
many instantiated pages into the process page tables as it sees
reasonable, while still obeying the correct semantic for COW and vm
map locking.

The method is opt-in, pager sets OBJ_POPULATE flag to indicate that
the method can be called. If pager' vm objects can be shadowed, pager
must implement the traditional getpages() method in addition to the
populate(). Populate() might fall back to the getpages() on per-call
basis as well, by returning VM_PAGER_BAD error code.

For now for device pagers, the populate() method is only allowed to be
used by the managed device pagers, but the limitation is only made
because there is no unmanaged fault handlers which could use it right
now.

KPI designed together with, and reviewed by: alc
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks