drm/vmwgfx: Refactor vmw_mksstat_remove_ioctl to expect pgid match with vmw_mksstat_a...

drm/vmwgfx: Refactor vmw_mksstat_remove_ioctl to expect pgid match with vmw_mksstat_add_ioctl to authorise removal.

Original vmw_mksstat_remove_ioctl expected pid to match the corresponding vmw_mksstat_add_ioctl.
That made impossible en-masse removals by one pid, which is a valid use case, so pid match was
discarded. Current change enforces a broader pgid match as a form of protection from arbitrary
processes interrupting an ongoing mks-guest-stats.

Reviewed-by: Zack Rusin <zackr@vmware.com>
Signed-off-by: Martin Krastev <krastevm@vmware.com>
Signed-off-by: Zack Rusin <zackr@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609172307.131929-8-zackr@vmware.com