Ext2: mark inode dirty after the function dquot_free_block_nodirty is called

We should mark inode dirty after the function dquot_free_block_nodirty
is called.Besides,add a check whether it is necessary to call
dquot_free_block_nodirty functon.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Jan Kara <jack@suse.cz>