ion: replace userspace handle cookies with idr

Userspace handles should not leak kernel virtual addresses to
userspace.  They have to be validated by looking them up in an
rbtree anyways, so replace them with an idr and validate them
by using idr_find to convert the id number to the struct
ion_handle pointer.

Change-Id: Iab5667ba6f3a73256fec3949b23a9a6f8e14a283
Signed-off-by: Colin Cross <ccross@android.com>
Git-commit: 1126d98d8e34d771afcc72ce5d1c37ad4d474402
Git-repo: https://android.googlesource.com/kernel/common


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