xfs: rt stubs should return negative errnos when rt disabled

When realtime support is not compiled into the kernel, these functions
should return negative errnos, not positive errnos.  While we're at it,
fix a broken macro declaration.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>