powerpc/xive: Add missing null check after calling kmalloc

Commit 930914b7d5 ("powerpc/xive: Add a debugfs file to dump
internal XIVE state") forgot to add a null check.

Add it.

Fixes: 930914b7d5 ("powerpc/xive: Add a debugfs file to dump internal XIVE state")
Signed-off-by: Ammar Faizi <ammarfaizi2@gmail.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211226135314.251221-1-ammar.faizi@intel.com

arch/powerpc/sysdev/xive/spapr.c

@@ -653,6 +653,9 @@ static int xive_spapr_debug_show(struct seq_file *m, void *private)
 	struct xive_irq_bitmap *xibm;
 	char *buf = kmalloc(PAGE_SIZE, GFP_KERNEL);
 
+	if (!buf)
+		return -ENOMEM;
+
 	list_for_each_entry(xibm, &xive_irq_bitmaps, list) {
 		memset(buf, 0, PAGE_SIZE);
 		bitmap_print_to_pagebuf(true, buf, xibm->bitmap, xibm->count);