x86/srso: Fix return thunks in generated code

stable inclusion
from stable-v5.10.189
commit 4873939c0e1cec2fd04a38ddf2c03a05e4eeb7ef
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I7RQ67
CVE: CVE-2023-20569

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4873939c0e1cec2fd04a38ddf2c03a05e4eeb7ef



--------------------------------

Upstream commit: 238ec850

Set X86_FEATURE_RETHUNK when enabling the SRSO mitigation so that
generated code (e.g., ftrace, static call, eBPF) generates "jmp
__x86_return_thunk" instead of RET.

  [ bp: Add a comment. ]

Fixes: fb3bd914 ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>