ueventd: Increase uevent buffer size

As it turns out, no, 64K is not enough. A device provisioning
tool was generating many uevents as it manipulated the GPT,
causing events near the end of the process (including events
as a result of the final BLKRRPART ioctl()) to get lost.

It's not clear what the best value for this should be, but
increasing to 256K fixed the problem in this case.

Change-Id: I4883b34e96c89e8a6fa581bc9cd121bb021b5694
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>