writeback: prep helpers for dirty-limit and writeback accounting

writeback: prep helpers for dirty-limit and writeback accounting

Add helper APIs needed by filesystems to avoid poking into writeback
internals.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>
Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://patch.msgid.link/20260213054634.79785-2-kundan.kumar@samsung.com
Signed-off-by: Christian Brauner <brauner@kernel.org>