[XFS] Use uninitialized_var macro to stop warning about rtx

Appease gcc in regards to "warning: 'rtx' is used uninitialized in
this function".

SGI-PV: 907752
SGI-Modid: xfs-linux-melb:xfs-kern:29007a

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>