Btrfs: make set/get functions for the super compat_ro flags use compat_ro

Our set/get functions for compat_ro_flags actually look at compat_flags.  This
will mess any attempt to use compat flags up.  The fix is obvious.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>