EX-8270 ptlrpc: remove PAGES_PER_POOL macro

EX-8270 ptlrpc: remove PAGES_PER_POOL macro

The page pool code *also* likes to refer to each page of
pointers it uses to track items in it as a "POOL", which is
incredibly confusing.

Start unwinding this by removing the PAGES_PER_POOL macro.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ie29434f53eeb945b8d35df7c1212ae3f51a2aafa
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52457
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>