parisc: fix compile warnings triggered by atomic_sub(sizeof(),v)

This fixes compile warnings like this one:
net/ipv4/igmp.c: In function ‘ip_mc_leave_group’:
net/ipv4/igmp.c:1898:3: warning: overflow in implicit constant conversion [-Woverflow]

atomic_sub() is defined as __atomic_add_return(-(VAL),(v))))
and if VAL is of type unsigned int (as returned by sizeof()), negating
this value will overflow. Fix this by type-casting VAL to int type.
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/include/asm/atomic.h

@@ -116,7 +116,7 @@ static __inline__ int __atomic_add_unless(atomic_t *v, int a, int u)
 
 
 #define atomic_add(i,v)	((void)(__atomic_add_return(        (i),(v))))
-#define atomic_sub(i,v)	((void)(__atomic_add_return(-(i),(v))))
+#define atomic_sub(i,v)	((void)(__atomic_add_return(-((int) (i)),(v))))
 #define atomic_inc(v)	((void)(__atomic_add_return(   1,(v))))
 #define atomic_dec(v)	((void)(__atomic_add_return(  -1,(v))))