EX-8270 ptlrpc: correct use of plural 'pools'

EX-8270 ptlrpc: correct use of plural 'pools'

There are a bunch of spots which refer to a single pool by
pool index, but which say 'pools'.  This is very confusing,
and in fact led to me misunderstanding the code at least
once.

Clean that up.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I9eabcfe77a57a82c87b36e3b3e040be91671fbfb
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52435
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>