dm io: don't call the async_io notify.fn on invalid num_regions

If dm_io() returned an error, callers that set a notify.fn and wanted it
called on an error need to check the return value and call notify.fn
themselves if it was -EINVAL but not if it was -EIO. None of them do
this (granted, all the existing async_io users of dm_io call it in a way
that is guaranteed to not return an error).

Simplify the interface by never calling the notify.fn if dm_io returns
an error. This works with the existing dm_io callers which check for an
error and handle it using the same methods as the notify.fn.

This also allows us to move the now equivalent num_regions checks out of
sync_io() and async_io() and into dm_io() itself. Additionally, change
async_io() into a void function, since it can no longer fail.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>