x86: Mark __invalid_creds() __noreturn

vmlinux.o: warning: objtool: ksys_unshare()+0x36c: unreachable instruction

0000 0000000000067040 <ksys_unshare>:
...
0364    673a4:	4c 89 ef             	mov    %r13,%rdi
0367    673a7:	e8 00 00 00 00       	call   673ac <ksys_unshare+0x36c>	673a8: R_X86_64_PLT32	__invalid_creds-0x4
036c    673ac:	e9 28 ff ff ff       	jmp    672d9 <ksys_unshare+0x299>
0371    673b1:	41 bc f4 ff ff ff    	mov    $0xfffffff4,%r12d
0377    673b7:	e9 80 fd ff ff       	jmp    6713c <ksys_unshare+0xfc>

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yi9gOW9f1GGwwUD6@hirez.programming.kicks-ass.net