Commit eab4e633 "xfs: uncached buffer reads need to return an error".

Remove redundant error variable, using the function level error variable
to store bp->b_error instead.
Signed-off-by: NJie Liu <jeff.liu@oracle.com>
Reviewed-by: NBen Myers <bpm@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>