lightnvm: don't check for failure from mempool_alloc()

mempool_alloc() cannot fail if the gfp flags allow it to
sleep, and both GFP_KERNEL and GFP_NOIO allows for sleeping.

So rrpc_move_valid_pages() and rrpc_make_rq() don't need to
test the return value.

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