EX-8270 ptlrpc: begin renaming pool_index to order

EX-8270 ptlrpc: begin renaming pool_index to order

Replace local variables for pool_index with pool_order.

Other renames will be in a subsequent patch, to keep these
as simple as possible.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: If347ff39776f9a75c0f7d9d9981d01e19bc2cbc9
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52453
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>