ia64: rwlock *_can_lock() primitives

Trivial but untested (but also currently unused - wait
for BKL PREEMPT to start using them soon).

include/asm-ia64/spinlock.h

@@ -126,6 +126,8 @@ typedef struct {
 #define RW_LOCK_UNLOCKED (rwlock_t) { 0, 0 }
 
 #define rwlock_init(x)		do { *(x) = RW_LOCK_UNLOCKED; } while(0)
+#define read_can_lock(rw)	(*(volatile int *)(rw) >= 0)
+#define write_can_lock(rw)	(*(volatile int *)(rw) == 0)
 
 #define _raw_read_lock(rw)								\
 do {											\