fs/xfs: Correct NULL test

Test the value that was just allocated rather than the previously tested one.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@r@
expression *x;
expression e;
identifier l;
@@

if (x == NULL || ...) {
    ... when forall
    return ...; }
... when != goto l;
    when != x = e
    when != &x
*x == NULL
// </smpl>
Signed-off-by: NJulia Lawall <julia@diku.dk>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAlex Elder <aelder@sgi.com>

fs/xfs/quota/xfs_qm.c

@@ -123,7 +123,7 @@ xfs_Gqm_init(void)
 		goto out;
 
 	gdqhash = kmem_zalloc_large(hsize);
-	if (!udqhash)
+	if (!gdqhash)
 		goto out_free_udqhash;
 
 	hsize /= sizeof(xfs_dqhash_t);