lightnvm: pblk: handle case when mw_cunits equals to 0

Some devices can expose mw_cunits equal to 0, it can cause the
creation of too small write buffer and cause performance to drop
on write workloads.

Additionally, write buffer size must cover write data requirements,
such as WS_MIN and MW_CUNITS - it must be greater than or equal to
the larger one multiplied by the number of PUs. However, for
performance reasons, use the WS_OPT value to calculation instead of
WS_MIN.

Because the place where buffer size is calculated was changed, this
patch also removes pgs_in_buffer filed in pblk structure.

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