LU-9090 ofd: increase default OST BRW size to 4MB

LU-9090 ofd: increase default OST BRW size to 4MB

Increase the default maximum bulk RPC size reported by OSTs to the
clients to 4MB.  This has shown to improve performance for tested
workloads under a variety of conditions, and is already the defacto
standard for new installations.

Clients will still negotiate their preferred and supported BRW size
with the OSTs at connection time.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Id213e598bde14ed53b78ec230c7ae9aa5f3ebbe5
Reviewed-on: https://review.whamcloud.com/25336
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>