Fix snapshot corruption in presence of concurrent updates introduced in 1.5.1-68

The final version of odnoklassniki patch did not include the
most important part - delayed free of salloc memory.

Make a quick & dirty fix to the point.

src/salloc.cc

@@ -45,6 +45,8 @@
 
 #define SLAB_ALIGN_PTR(ptr) (void *)((uintptr_t)(ptr) & ~(SLAB_SIZE - 1))
 
+extern int snapshot_pid;
+
 #ifdef SLAB_DEBUG
 #undef NDEBUG
 uint8_t red_zone[4] = { 0xfa, 0xfa, 0xfa, 0xfa };
@@ -322,6 +324,9 @@ sfree(void *ptr)
 static void
 sfree_batch(void)
 {
+	if (snapshot_pid)
+		return;
+
 	ssize_t batch = arena.delayed_free_batch;
 
 	while (--batch >= 0 && !SLIST_EMPTY(&free_delayed)) {

src/tarantool.cc

@@ -324,8 +324,6 @@ snapshot(void)
 	if (snapshot_pid)
 		return EINPROGRESS;
 
-	/* increment snapshot version */
-	snapshot_version++;
 
 	pid_t p = fork();
 	if (p < 0) {
@@ -334,6 +332,8 @@ snapshot(void)
 	}
 	if (p > 0) {
 		snapshot_pid = p;
+		/* increment snapshot version */
+		snapshot_version++;
 		int status = wait_for_child(p);
 		snapshot_pid = 0;
 		return (WIFSIGNALED(status) ? EINTR : WEXITSTATUS(status));