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

In the Linux kernel, the following vulnerability has been resolved:

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.