ftl: fix off by one in garbage collection code

source/ftl/ftl.c

@@ -543,10 +543,10 @@ static int free_tail(struct ftl_journal *j)
 	size_t npages = j->nblocks << log2_pages_per_block;
 	size_t dist;
 
-	if (j->tail < j->head)
-		dist = npages - j->head + j->tail;
-	else
+	if (j->head < j->tail)
 		dist = j->tail - j->head;
+	else
+		dist = npages - j->head + j->tail;
 
 	if (dist > (UINT32_C(1) << log2_pages_per_block))
 		return 0;