IB/core: Expose ioctl interface through experimental Kconfig

Add CONFIG_INFINIBAND_EXP_USER_ACCESS that enables the ioctl
interface. This interface is experimental and is subject to change.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>