um: Add missing apply_returns()

stable inclusion
from stable-v5.10.133
commit 8e2774270aa319f552fab969bc4500e412d2566b
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5PTAS
CVE: CVE-2022-29900,CVE-2022-23816,CVE-2022-29901

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8e2774270aa319f552fab969bc4500e412d2566b

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

commit 564d9981 upstream.

Implement apply_returns() stub for UM, just like all the other patching
routines.

Fixes: 15e67227 ("x86: Undo return-thunk damage")
Reported-by: Randy Dunlap <rdunlap@infradead.org)
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/Ys%2Ft45l%2FgarIrD0u@worktop.programming.kicks-ass.netSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLin Yujun <linyujun809@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

arch/um/kernel/um_arch.c

@@ -362,6 +362,10 @@ void apply_retpolines(s32 *start, s32 *end)
 {
 }
 
+void apply_returns(s32 *start, s32 *end)
+{
+}
+
 void apply_alternatives(struct alt_instr *start, struct alt_instr *end)
 {
 }