EX-8270 ptlrpc: stop passing around pool_index

EX-8270 ptlrpc: stop passing around pool_index

We pass pool_index around from function to function over
and over, but it's easier to just pass the pool around.

This does require the pool to know its own index, but
that seems better anyway.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I42dc8b8094212c69b7a29cc3766bd0a10860f7af
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52437
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>