x86/ioperm: Initialize pointer bitmap with NULL rather than 0

The pointer bitmap is being initialized with a plain integer 0,
fix this by initializing it with a NULL instead.

Cleans up sparse warning:
arch/x86/xen/enlighten_pv.c:876:27: warning: Using plain integer
as NULL pointer
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200721100217.407975-1-colin.king@canonical.com

arch/x86/xen/enlighten_pv.c

@@ -873,7 +873,7 @@ static void xen_load_sp0(unsigned long sp0)
 static void xen_invalidate_io_bitmap(void)
 {
 	struct physdev_set_iobitmap iobitmap = {
-		.bitmap = 0,
+		.bitmap = NULL,
 		.nr_ports = 0,
 	};