EX-8270 ptlrpc: rename 'pool' to 'pool_idx'

EX-8270 ptlrpc: rename 'pool' to 'pool_idx'

'pool' here is the index of the pool, not the pool itself.
Let's give it a name that makes clear it's a number and not
the actual pool.

Also remove an error condition which is asserted on
immediately before.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I636a4756a033d0b96a4772b8912f61c4b31b9c64
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52426
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>