[PATCH 0/2 V6]Add dma-buf counter

[Peter Enderborg]

The dma-buf counter is a metric for mapped memory used by it's clients.
It is a shared buffer that is typically used for interprocess communication
or process to hardware communication. In android we used to have ION,. but
it is now replaced with dma-buf. ION had some overview metrics that was similar.



V1
	initial version. Add dma-buf counter

V2
	Fix build depencendy error suggested by Matthew Wilcox
	Extent commit message sugged by Köning

V3
	Change variable and function names.

V4
	Fix function name in code doc
	Reported-by: kernel test robot <lkp at intel.com>

V5
	Removed EXPORT_SYMBOL_GPL suggested by Muchun Song

V6
	Made it a patch set, Adding a addional patch for
	printing dma-buf counter in show_mem.
	Suggested by Michal Hocko.