LU-3606 lustre: Reserve OST_FALLOCATE(fallocate) opcode

LU-3606 lustre: Reserve OST_FALLOCATE(fallocate) opcode

A new RPC, OST_FALLOCATE has been added for
space preallocation. This patch reserves
OST_FALLOCATE opcode for fallocate syscall.
Reserving opcode upfront would ensure consistency
and would avoid protocol interoperability issues
in the future.

Test-Parameters: trivial testlist=sanity,sanityn,sanity-dom
Signed-off-by: Swapnil Pimpale <spimpale@ddn.com>
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Abrarahmed Momin <abrar.momin@gmail.com>
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Change-Id: Ie109f8f5720dec6d34c5ce4f7732fe49ccb47cd9
Reviewed-on: https://review.whamcloud.com/37277
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>