LU-1431 ptlrpc: PTLRPC_BRW_MAX_SIZE usage cleanup.

Clean-up the layering in current code by eliminating direct
usage of PTLRPC_BRW_MAX_SIZE macro outside of the ptlrpc
module. This should help us acheive "floating" max brw size
value across the cluster, which in turn should help with
4MB IO task.

Signed-off-by: Sergii Glushchenko <sergii_glushchenko@xyratex.com>
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-by: Alexander Zarochentsev <Alexander_Zarochentsev@xyratex.com>
Change-Id: Ib9f6265bd28f25c34b27eb6ad1a56a752b71a4ac
Xyratex-bug-id: MRP-687
Reviewed-on: http://review.whamcloud.com/4876
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>