writeback: add @gfp to wb_init()

wb_init() currently always uses GFP_KERNEL but the planned cgroup
writeback support needs using other allocation masks.  Add @gfp to
wb_init().

This patch doesn't introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@fb.com>