writeback: fix initial dirty limit

The initial value of global_wb_domain.dirty_limit set by
writeback_set_ratelimit() is zeroed out by the memset in
wb_domain_init().

Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>