ubifs: Fix wrong dirty space budget for dirty inode

Each dirty inode should reserve 'c->bi.inode_budget' bytes in space
budget calculation. Currently, space budget for dirty inode reports
more space than what UBIFS actually needs to write.

Fixes: 1e51764a ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>