ext4: cleanup in ext4_discard_allocated_blocks()

remove 'len' variable in ext4_discard_allocated_blocks() because it is
useless.
Signed-off-by: NZheng Liu <wenqing.lz@taobao.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -3073,13 +3073,9 @@ static void ext4_mb_collect_stats(struct ext4_allocation_context *ac)
 static void ext4_discard_allocated_blocks(struct ext4_allocation_context *ac)
 {
 	struct ext4_prealloc_space *pa = ac->ac_pa;
-	int len;
-
-	if (pa && pa->pa_type == MB_INODE_PA) {
-		len = ac->ac_b_ex.fe_len;
-		pa->pa_free += len;
-	}
 
+	if (pa && pa->pa_type == MB_INODE_PA)
+		pa->pa_free += ac->ac_b_ex.fe_len;
 }
 
 /*