staging: lustre: osc: fix bug when setting max_pages_per_rpc

staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6421
Reviewed-on: http://review.whamcloud.com/14333
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>