LU-13814 osc: clarify osc_transfer_pinned usage

LU-13814 osc: clarify osc_transfer_pinned usage

osc_transfer_pinned is not used for transient pages, so
don't clear it for them either.

This is part of a process of cleaning up DIO page
submission by truly specializing it for DIO pages.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I48f0da5f3cede5af04770c64077bbe7475623881
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52139
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>