locking/atomic: cmpxchg: Make __generic_cmpxchg_local compare against zero-extended 'old' value

__generic_cmpxchg_local takes unsigned long old/new arguments which
might end up being up-cast from smaller signed types (which will
sign-extend).  The loaded compare value must be compared against a
truncated smaller type, so down-cast appropriately for each size.

The issue is apparent on 64-bit machines with code, such as
atomic_dec_unless_positive(), that sign-extends from int.

64-bit machines generally don't use the generic cmpxchg but
development/early ports might make use of it, so make it correct.
Signed-off-by: Matt Evans <mev@rivosinc.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/cmpxchg-local.h

@@ -26,15 +26,15 @@ static inline unsigned long __generic_cmpxchg_local(volatile void *ptr,
 	raw_local_irq_save(flags);
 	switch (size) {
 	case 1: prev = *(u8 *)ptr;
-		if (prev == old)
+		if (prev == (u8)old)
 			*(u8 *)ptr = (u8)new;
 		break;
 	case 2: prev = *(u16 *)ptr;
-		if (prev == old)
+		if (prev == (u16)old)
 			*(u16 *)ptr = (u16)new;
 		break;
 	case 4: prev = *(u32 *)ptr;
-		if (prev == old)
+		if (prev == (u32)old)
 			*(u32 *)ptr = (u32)new;
 		break;
 	case 8: prev = *(u64 *)ptr;