[kernel] Make sure high bits are 0 writing SFMASK MSR

QEMU handles bits bring written in the (reserved) high bits of SFMASK
just fine, but KVM gives a #GP exception.

src/kernel/syscall.s

@@ -116,6 +116,7 @@ syscall_enable:
 
 	mov rcx, MSR_FMASK
 	mov rax, FMASK_VAL
+	mov rdx, 0
 	wrmsr
 
 	pop rbp