Documentation/litmus-tests: Make cmpxchg() tests safe for klitmus

The four litmus tests in Documentation/litmus-tests/atomic do not
declare all of their local variables. Although this is just fine for LKMM
analysis by herd7, it causes build failures when run in-kernel by klitmus.
This commit therefore adjusts these tests to declare all local variables.

Reported-by: Andrea Parri <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Acked-by: Andrea Parri <[e-mail]>

Documentation/litmus-tests/atomic/cmpxchg-fail-ordered-1.litmus

···
 P1(int *x, int *y, int *z)
 {
 	int r0;
+	int r1;
 
 	WRITE_ONCE(*y, 1);
 	r1 = cmpxchg(z, 1, 0);

Documentation/litmus-tests/atomic/cmpxchg-fail-ordered-2.litmus

···
 
 P0(int *x, int *y)
 {
-	int r0;
 	int r1;
 
 	WRITE_ONCE(*x, 1);
···
 
 P1(int *x, int *y)
 {
-	int r0;
+	int r1;
+	int r2;
 
 	r1 = cmpxchg(y, 0, 1);
 	smp_mb__after_atomic();

Documentation/litmus-tests/atomic/cmpxchg-fail-unordered-1.litmus

···
 P1(int *x, int *y, int *z)
 {
 	int r0;
+	int r1;
 
 	WRITE_ONCE(*y, 1);
 	r1 = cmpxchg(z, 1, 0);

Documentation/litmus-tests/atomic/cmpxchg-fail-unordered-2.litmus

···
 
 P0(int *x, int *y)
 {
-	int r0;
 	int r1;
 
 	WRITE_ONCE(*x, 1);
···
 
 P1(int *x, int *y)
 {
-	int r0;
+	int r1;
+	int r2;
 
 	r1 = cmpxchg(y, 0, 1);
 	r2 = READ_ONCE(*x);