btrfs: scrub: remove the force parameter from scrub_pages

The @force parameter for scrub_pages() is to indicate whether we want to
force bio submission.  Currently it's only used for the super block,
and it can be easily determined by the @flags, so we can remove the
parameter.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>