kdbus: increase kdbus receive pool size to 16M

kdbus: increase kdbus receive pool size to 16M

Current kdbus receive pool size is too small (only 2MB - in GLib
we have 16MB). Due to quite specific 'pool layout' in kdbus module,
2MB was not enough for 512kB messages - according to kdbus docs:

"50% of a pool is always owned by the connection. It is reserved for
kernel queries, handling received messages and other tasks that are
under control of the pool owner. The other 50% of the pool are used
as incoming queue.
As we optionally support user-space based policies, we need fair
allocation schemes. Furthermore, resource utilization should be
maximized, so only minimal resources stay reserved. However, we need
to adapt to a dynamic number of users, as we cannot know how many
users will talk to a connection. Therefore, the current allocation
works like this:
We limit the number of bytes in a destination's pool per sending
user. The space available for a user is 33% of the unused pool space
(whereas the space used by the user itself is also treated as
'unused'). This way, we favor users coming first, but keep enough
pool space available for any following users. Given that messages are
dequeued in FIFO order, this should balance nicely if the number of
users grows. At the same time, this algorithm guarantees that the
space available to a connection is reduced dynamically, the more
concurrent users talk to a connection."

Change-Id: Iffddc018f0d8bf08451d12d305c8d392ed3e1f55