libata: add command iterator helpers

Now that we have the internal tag as a special (higher) value tag,
it gets a bit tricky to iterate the internal commands as some loops
will exceed ATA_MAX_QUEUE. Add explicit helpers for iterating pending
commands, both inflight and internal.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Tejun Heo <tj@kernel.org>