writeback: make __wb_writeout_inc() and hard_dirty_limit() take wb_domaas a parameter
Currently __wb_writeout_inc() and hard_dirty_limit() assume
global_wb_domain; however, cgroup writeback support requires
considering per-memcg wb_domain too.
This patch separates out domain-specific part of __wb_writeout_inc()
into wb_domain_writeout_inc() which takes wb_domain as a parameter and
adds the parameter to hard_dirty_limit(). This will allow these two
functions to handle per-memcg wb_domains.
This patch doesn't introduce any behavioral changes.