android: binder: add extra size to allocator.

The binder_buffer allocator currently only allocates
space for the data and offsets buffers of a Parcel.
This change allows for requesting an additional chunk
of data in the buffer, which can for example be used
to hold additional meta-data about the transaction
(eg a security context).

Change-Id: I58ab9c383a2e1a3057aae6adaa596ce867f1b157
Signed-off-by: Martijn Coenen <maco@google.com>
Git-commit: 6bae6a826b5afa337fa7df90c4373d9a5f66ed4f
Git-repo: https://android.googlesource.com/kernel/common


Signed-off-by: Srinivasarao P <spathi@codeaurora.org>