x86,static_call: Fix __static_call_return0 for i386

Paolo reported that the instruction sequence that is used to replace:

    call __static_call_return0

namely:

    66 66 48 31 c0	data16 data16 xor %rax,%rax

decodes to something else on i386, namely:

    66 66 48		data16 dec %ax
    31 c0		xor    %eax,%eax

Which is a nonsensical sequence that happens to have the same outcome.
*However* an important distinction is that it consists of 2
instructions which is a problem when the thing needs to be overwriten
with a regular call instruction again.

As such, replace the instruction with something that decodes the same
on both i386 and x86_64.

Fixes: 3f2a8fc ("static_call/x86: Add __static_call_return0()")
Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220318204419.GT8939@worktop.programming.kicks-ass.net

Author: Peter Zijlstra

arch/x86/kernel/static_call.c

@@ -12,10 +12,9 @@ enum insn_type {
 };
 
 /*
- * data16 data16 xorq %rax, %rax - a single 5 byte instruction that clears %rax
- * The REX.W cancels the effect of any data16.
+ * cs cs cs xorl %eax, %eax - a single 5 byte instruction that clears %[er]ax
  */
-static const u8 xor5rax[] = { 0x66, 0x66, 0x48, 0x31, 0xc0 };
+static const u8 xor5rax[] = { 0x2e, 0x2e, 0x2e, 0x31, 0xc0 };
 
 static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };