ftl: fix off by one in garbage collection code

source/ftl/ftl.c

@@ -540,13 +540,13 @@ static int free_tail(struct ftl_journal *j)
 {
 	size_t log2_pages_per_block = j->log2_pages_per_group +
 		j->log2_groups_per_block;
-	size_t npages = j->nblocks << log2_pages_per_block;	
+	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;