EX-7389 pcc: open file for detach O_RDONLY

EX-7389 pcc: open file for detach O_RDONLY

llapi_pcc_detach_file is rdwr, but should just be rdonly.
This means files can be attached but not detached if the
client is mounted rdonly.

The fix is just to open the file in detach with O_RDONLY.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I3e289ab52ff760a8ab84a209b968109517953b52
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50749
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>