xfs: enable the finobt feature on v5 superblocks

Add the finobt feature bit to the list of known features. As of
this point, the kernel code knows how to mount and manage both
finobt and non-finobt formatted filesystems.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>