Implement apply_returns() stub for UM, just like all the other patching
routines.
Fixes: 15e67227c4 ("x86: Undo return-thunk damage")
Reported-by: Randy Dunlap <rdunlap@infradead.org)
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>