ion: index client->handles rbtree by buffer

The only remaining users of the client->handles rbtree are
iterating through it like a list.  Keep the rbtree, but change
its index to be the buffer address instead of the handle address,
which makes ion_handle_lookup a fast rbtree search.

Change-Id: Ie7d974b3a5d9831c0d664de85ddae8db3c3abdf9
Signed-off-by: Colin Cross <ccross@android.com>
Git-commit: 002119eeb221cf1efd7afda836da768ff1cd0ad9
Git-repo: http://android.googlesource.com/kernel/common/


Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>