Add a hook to vfio_device_ops to allow sub-modules provide virtual
addresses for an mmap() request.
Note that the fallback will be mm_get_unmapped_area(), which should
maintain the old behavior of generic VA allocation (__get_unmapped_area).
It's a bit unfortunate that is needed, as the current get_unmapped_area()
file ops cannot support a retval which fallbacks to the default. So that
is needed both here and whenever sub-module will opt-in with its own.
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---