From: Nick Piggin <piggin@cyberone.com.au>

The batch_requests function got lost during the merge of the dynamic request
allocation patch.

We need it for the anticipatory scheduler - when the number of threads
exceeds the number of requests, the anticipated-upon task will undesirably
sleep in get_request_wait().

And apparently some block devices which use small requests need it so they
string a decent number together.

Jens has acked this patch.