Check for SQEs, not for completions

Should finally fix Assertion `sqe != NULL' failed introduced after journaling
refactor in 0.6.11...

src/ringloop.cpp

@@ -112,3 +112,11 @@ void ring_loop_t::restore(unsigned sqe_tail)
     }
     ring.sq.sqe_tail = sqe_tail;
 }
+
+int ring_loop_t::sqes_left()
+{
+    struct io_uring_sq *sq = &ring.sq;
+    unsigned int head = io_uring_smp_load_acquire(sq->khead);
+    unsigned int next = sq->sqe_tail + 1;
+    return *sq->kring_entries - (next - head);
+}