Make the framework for reporting per-process memory statistics a little bit
more generic. This should make it easier to keep track of more external
memory sources as they are added.
The existing timestamp_cmp function returns a different
result depending on the order of the input parameters due to
having an asymetric valid window. When no rollover is
detected the window is 2^31 but when a rollover is detected
the window is 25000. This change makes the rollover window
symmetric at 2^31.
Some hangs are fooling the postmortem dump code into
running off the end of a buffer. Fix this by making
its bounds check logic work better by reusing the
logic from kgsl_find_region().
