x86/xen: Remove use of VLAs

There's an ongoing effort to remove VLAs[1] from the kernel to eventually
turn on -Wvla. It turns out, the few VLAs in use in Xen produce only a
single entry array that is always bounded by GDT_SIZE. Clean up the code to
get rid of the VLA and the loop.

[1] https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

[boris: Use BUG_ON(size>PAGE_SIZE) instead of GDT_SIZE]
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>