btrfs: scrub: convert scrub_workers_refcnt to refcount_t

Use the refcount_t for fs_info::scrub_workers_refcnt instead of int so
we get the extra checks. All reference changes are still done under
scrub_lock.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>