LU-15619 osc: Rename brw_page members

LU-15619 osc: Rename brw_page members

The brw_page members have generic names - add a structure
related tag to the names.

test-parameters: trivial

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I98e6f874902074934eb01476a9595f502526bc38
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/46715
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>