mm/page-writeback.c: Fix comment of wb_domain_writeout_add()

__bdi_writeout_inc() has undergone multiple renamings, but the comment
within the function body have not been updated accordingly. Update it
to reflect the latest wb_domain_writeout_add().

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Link: https://lore.kernel.org/r/20241009151728.300477-3-yizhou.tang@shopee.com


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>