lightnvm: do not free unused metadata on rrpc

rrpc does not save any metadata on a given request. Thus, do not attempt
to free the metadata dma region.

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