EX-8270 ptlrpc: change "pool" to "page_ptrs"

EX-8270 ptlrpc: change "pool" to "page_ptrs"

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.

This patch works on renaming that to page_ptrs, but leaves
some steps for a future patch.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I56ee54c7f39b52d7cceffec9e3decf71bd313ddc
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52458
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>