staging: android: ion: Add per-heap counters

Heap statistics have been removed and currently even basics statistics
are missing.

This patch creates per heap debugfs directory /sys/kernel/debug/<heap_name>
and adds the following counters:
- the number of allocated buffers;
- the number of allocated bytes;
- the number of allocated bytes watermark.

Signed-off-by: Alexey Skidanov <alexey.skidanov@intel.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>