[PATCH 45/49] writeback: make writeback initiation functions handle multiple bdi_writeback's

[try_]writeback_inodes_sb[_nr]() and sync_inodes_sb() currently only handle dirty inodes on the root wb (bdi_writeback) of the target bdi. This patch implements bdi_split_work_to_wbs() and use it to make these functions handle multiple wb's. bdi_split_work_to_wbs() takes a base wb_writeback_work

[PATCH 45/49] writeback: make writeback initiation functions handle multiple bdi_writeback's

[try_]writeback_inodes_sb[_nr]() and sync_inodes_sb() currently only handle dirty inodes on the root wb (bdi_writeback) of the target bdi. This patch implements bdi_split_work_to_wbs() and use it to make these functions handle multiple wb's. bdi_split_work_to_wbs() takes a base wb_writeback_work