lightnvm: submit erases using the I/O path

Until now erases have been submitted as synchronous commands through a
dedicated erase function. In order to enable targets implementing
asynchronous erases, refactor the erase path so that it uses the normal
async I/O submission functions. If a target requires sync I/O, it can
implement it internally. Also, adapt rrpc to use the new erase path.

Signed-off-by: Javier González <javier@cnexlabs.com>
Fixed spelling error.
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>

Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>