um: Add missing apply_returns()

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: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>