xfs: fix getfsmap reporting past the last rt extent

The realtime section ends at the last rt extent.  If the user configures
the rt geometry with an extent size that is not an integer factor of the
number of rt blocks, it's possible for there to be rt blocks past the
end of the last rt extent.  These tail blocks cannot ever be allocated
and will cause corruption reports if the last extent coincides with the
end of an rt bitmap block, so do not report consider them for the
GETFSMAP output.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>