Kernel: Make all syscall functions return KResultOr<T>

This makes it a lot easier to return errors since we no longer have to
worry about negating EFOO errors and can just return them flat.

Kernel/Syscalls/perf_event.cpp

@@ -29,7 +29,7 @@
 
 namespace Kernel {
 
-int Process::sys$perf_event(int type, FlatPtr arg1, FlatPtr arg2)
+KResultOr<int> Process::sys$perf_event(int type, FlatPtr arg1, FlatPtr arg2)
 {
     return ensure_perf_events().append(type, arg1, arg2);
 }