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: Iabf59a64ff66a0457a9aa7077aef219c3dc05ba8
Signed-off-by: Martijn Coenen <maco@google.com>
Git-commit: d7c114e9
Git-repo: https://git.linaro.org/kernel/linux-linaro-stable


Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>