LU-2756 ost: buffer utilization rate of OST rqbd

This patch covered a couple of things:
- different request buffer size for different OST services
  only requests on OST_IO_PORTAL need large buffers

- add extra 46K to OST_IO_BUFSIZE (buffer size is increased to 64K)
  size of OST_IO_MAXREQSIZE is about 18K, if OST_IO_BUFSIZE is only
  a little bigger than OST_IO_MAXREQSIZE, then each rqbd can only fit
  in 2-3 request even there are 18K bytes left in this rqbd, memory
  utilization rate is low. Extra bytes can make buffer utilization
  rate increase to 72%. Also, it can avoid version compatibility
  issue in the future if we want to increase request size again.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Change-Id: Iaf8b45520b000dd5219a4af2ec3f5b2ea7f53134
Reviewed-on: http://review.whamcloud.com/5626
Tested-by: Hudson
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>