Is UBIFS need backgroud gc?

[wang.bo116 at zte.com.cn]

	UBIFS now use ubifs_budget_space to ensure there is enough space to complete an operation.
When an operation cannot be budgeted, make_free_space -> run_gc called to make some free space.
	My questions is that why not attempt run gc in backgroud when UBIFS is in idle? Do gc
when an operation cannot be budgeted is too late, and this may block the operation for a while.
	Maybe now it is time to add backgroud gc in ubifs_bg_thread?