Check for SQEs, not for completions

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

src/blockstore_impl.h

@@ -55,7 +55,7 @@
 #define IS_DELETE(st) (((st) & 0x0F) == BS_ST_DELETE)
 
 #define BS_SUBMIT_CHECK_SQES(n) \
-    if (ringloop->space_left() < (n))\
+    if (ringloop->sqes_left() < (n))\
     {\
         /* Pause until there are more requests available */\
         PRIV(op)->wait_for = WAIT_SQE;\

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);
+}

src/ringloop.h

@@ -172,6 +172,7 @@ public:
         struct io_uring_cqe *cqe;
         return io_uring_wait_cqe(&ring, &cqe);
     }
+    int sqes_left();
     inline unsigned space_left()
     {
         return free_ring_data_ptr;