ext4: let ext4_group_add_blocks() handle 0 blocks quickly

If ext4_group_add_blocks() is called with 0 block, make it return 0
without doing any extra work.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>