ion: convert map_kernel to return ERR_PTR

ion is going to stop accepting NULL as an error value, use ERR_PTR.

Signed-off-by: Colin Cross <ccross@android.com>
[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>