dm table: allow targets without devices to set ->io_hints

In dm_calculate_queue_limits, add call to ->io_hints hook if the
target doesn't provide ->iterate_devices.

This is needed so the "error" and "zero" targets may support
discards. The 2 following commits will add their respective discard
support.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>