quoted

Quite some additional newlines and I'm not really fond of the
->poll_batch() name. It's a bit confusing with ->poll() and we also have
irq_poll(). But the only thing that would come to my mind is
complete_batch() which "races" with ->complete().

What about ->check_completions()? After all, that is what both ->poll()
and ->poll_batch do but with a different stop condition, no ?
So it would also be easy to merge the two: both tag and batch are
unsigned int which could be called "cookie", and add a flag to tell how
to interpret it (as a tag or a batch limit).
e.g. something like:

+typedef int (check_completions_fn)(struct blk_mq_hw_ctx *,
		enum blk_mq_check_flags, /* flag (TAG or BATCH) */
		unsigned int); /* Target tag or batch limit */

I'd rather not to unite poll/poll_batch, but if this is something
that people want I can definitely do it.