spinlock/dec: backoff until lock state transition in lock_eb.

This fixes an issue caught by Changwoo Min (@multics69).

include/spinlock/dec.h

@@ -111,6 +111,7 @@ ck_spinlock_dec_lock_eb(struct ck_spinlock_dec *lock)
 		if (r == true)
 			break;
 
+		while (ck_pr_load_uint(&lock->value) != 1)
 			ck_backoff_eb(&backoff);
 	}