lightnvm: pblk: add possibility to set write buffer size manually

In some cases, users can want set write buffer size manually, e.g. to
adjust it to specific workload. This patch provides the possibility
to set write buffer size via module parameter feature.

Signed-off-by: Marcin Dziegielewski <marcin.dziegielewski@intel.com>
Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>