x86-64: rwlock *_can_lock() primitives

include/asm-x86_64/spinlock.h

@@ -159,6 +159,9 @@ typedef struct {
 
 #define rwlock_init(x)	do { *(x) = RW_LOCK_UNLOCKED; } while(0)
 
+#define read_can_lock(x)	((int)(x)->lock > 0)
+#define write_can_lock(x)	((x)->lock == RW_LOCK_BIAS)
+
 /*
  * On x86, we implement read-write locks as a 32-bit counter
  * with the high bit (sign) being the "contended" bit.