nvme: make nvme_get_log_ext non-static

Enable the lightnvm integration to use the nvme_get_log_ext()
function.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>