LU-17422 obdclass: rename ptlrpc_page_pool

LU-17422 obdclass: rename ptlrpc_page_pool

This patch renames the ptlrpc page pool to reflect its new
place in obd.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <patrick.farrell@oracle.com>
Change-Id: I67aa5f3eef26b5fb890e62bced837bea9dd032c6
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53668
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>