lightnvm: introduce max_phys_sects helper function

Target devices do not have access to the device driver operations.
Introduce a helper function that exposes the max. number of physical
sectors supported by the underlying device.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>