dm: add support for REQ_NOWAIT and enable it for linear target

Add DM target feature flag DM_TARGET_NOWAIT which advertises that
target works with REQ_NOWAIT bios.

Add dm_table_supports_nowait() and update dm_table_set_restrictions()
to set/clear QUEUE_FLAG_NOWAIT accordingly.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>