atomic: fix atomic_cas* comment documenting the behavior

It was backwards from reality (and tests).  Returning the "before" value is
more useful because it allows us to retry the CAS without having to reload
the variable to get the old value.

Signed-off-by: Josef 'Jeff' Sipek <[email protected]>

Author: jeffpc

include/jeffpc/atomic.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2019 Josef 'Jeff' Sipek <[email protected]>
+ * Copyright (c) 2014-2020 Josef 'Jeff' Sipek <[email protected]>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -49,7 +49,7 @@ typedef struct {
 #define atomic_inc(var)		atomic_add((var), 1)
 #define atomic_dec(var)		atomic_sub((var), 1)
 
-/* The following return the 'after' value.  If == new, then a swap occured. */
+/* The following return the 'before' value.  If == old, then a swap occured. */
 #define atomic_cas(var, old, new)	\
 		__sync_val_compare_and_swap(&(var)->v, (old), (new))
 #define atomic_cas_ptr(var, old, new)	\