RDMA/nes: Add a check for strict_strtoul()

It should check if strict_strtoul() succeeds before using
'wqm_quanta_value'.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>

[ Convert to kstrtoul() directly while we're here.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>