x86/entry: __always_inline CR2 for noinstr

vmlinux.o: warning: objtool: exc_page_fault()+0x9: call to read_cr2() leaves .noinstr.text section
vmlinux.o: warning: objtool: exc_page_fault()+0x24: call to prefetchw() leaves .noinstr.text section
vmlinux.o: warning: objtool: exc_page_fault()+0x21: call to kvm_handle_async_pf.isra.0() leaves .noinstr.text section
vmlinux.o: warning: objtool: exc_nmi()+0x1cc: call to write_cr2() leaves .noinstr.text section

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200603114052.243227806@infradead.org