LU-6421 osc: fix bug when setting max_pages_per_rpc

LU-6421 osc: fix bug when setting max_pages_per_rpc

After setting like "lctl set_param -P osc.*.max_pages_per_rpc", it
is possible that the function osc_obd_max_pages_per_rpc_seq_write
will be called before ocd_brw_size has been set when mount.
ocd_brw_size is meaningless when it is zero. So it should not be
the limit at that time.

Signed-off-by: Wu Libin <lwu@ddn.com>
Change-Id: I1abbd1bfc089ca1fea2b7ec71a6d45ee70f1ba20
Reviewed-on: http://review.whamcloud.com/14333
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>