LU-8135 osc: limits the number of chunks in write RPC

LU-8135 osc: limits the number of chunks in write RPC

OSC has to make sure that it won't issue write RPCs with too many
chunks otherwise it will casue ZFS to create transactions much
bigger than DMU_MAX_ACCESS in size, which will end up with write
failure.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ib68b09afca35c253ef0a6b569f64f555e08bd11b
Reviewed-on: http://review.whamcloud.com/22369
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>