xen/privcmd: Fix a possible warning in privcmd_ioctl_mmap_resource()

As 'kdata.num' is user-controlled data, if user tries to allocate
memory larger than(>=) MAX_ORDER, then kcalloc() will fail, it
creates a stack trace and messes up dmesg with a warning.

Call trace:
-> privcmd_ioctl
--> privcmd_ioctl_mmap_resource

Add __GFP_NOWARN in order to avoid too large allocation warning.
This is detected by static analysis using smatch.

Fixes: 3ad08765 ("xen/privcmd: add IOCTL_PRIVCMD_MMAP_RESOURCE")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20221126050745.778967-1-harshit.m.mogalapalli@oracle.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/privcmd.c

@@ -760,7 +760,7 @@ static long privcmd_ioctl_mmap_resource(struct file *file,
 		goto out;
 	}
 
-	pfns = kcalloc(kdata.num, sizeof(*pfns), GFP_KERNEL);
+	pfns = kcalloc(kdata.num, sizeof(*pfns), GFP_KERNEL | __GFP_NOWARN);
 	if (!pfns) {
 		rc = -ENOMEM;
 		goto out;