writeback: make wb_writeback() take an argument structure

We need to be able to pass in range_cyclic as well, so instead
of growing yet another argument, split the arguments into a
struct wb_writeback_args structure that we can use internally.
Also makes it easier to just copy all members to an on-stack
struct, since we can't access work after clearing the pending
bit.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>