writeback: implement bdi_for_each_wb()
writeback: implement bdi_for_each_wb()

This will be used to implement bdi-wide operations which should be
distributed across all its cgroup bdi_writebacks.

Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>
Cc: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>
Cc: Jan Kara <596727c8a0ea4db3ba2ceceedccbacd3d7b371b8@suse.cz>
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@fb.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs