ion: add new ion_user_handle_t type for the user-space token

Declare new ion_user_handle_t type to contain the token returned to user-space.
This allows a 2-step migration of the user-space code to a new kernel header
first, then will allow us to change the definition of the ion_user_handle_type_t
to int without breaking the API.

Signed-off-by: Rom Lemarchand <romlem@google.com>
(cherry picked from commit ebb8269bbb05b06ecedca3e21b3e65f23d48eadd)
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>