ion: convert map_kernel to return ERR_PTR

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

Change-Id: I030e8b72138904e38a4a5d225beaaa98427651fb
Signed-off-by: Colin Cross <ccross@android.com>
Git-commit: d37aa151f874563a35d49a0ff0a129ef332c5e9e
Git-repo: http://android.googlesource.com/kernel/common/


[mitchelh@codeaurora.org: conflicts due to msm changes]
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>