IB/core: Re-enable create_flow/destroy_flow uverbs

This commit reverts commit 7afbddfa ("IB/core: Temporarily disable
create_flow/destroy_flow uverbs").  Since the uverbs extensions
functionality was experimental for v3.12, this patch re-enables the
support for them and flow-steering for v3.13.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>