um: Add missing apply_returns()

um: Add missing apply_returns()

commit 8eb48ab735cb5c73525a5310348c1ce4cd54066b upstream.

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

Fixes: f27938f128c2 ("x86: Undo return-thunk damage")
Reported-by: Randy Dunlap <rdunlap@infradead.org)
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/Ys%2Ft45l%2FgarIrD0u@worktop.programming.kicks-ass.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>