x86/xen: Make write_cr2() noinstr

vmlinux.o: warning: objtool: pv_ops[42]: native_write_cr2
vmlinux.o: warning: objtool: pv_ops[42]: xen_write_cr2
vmlinux.o: warning: objtool: exc_nmi()+0x127: call to pv_ops[42]() leaves .noinstr.text section

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210624095148.563524913@infradead.org