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The sample structure contains the field 'data_src' which is used to
tell the data operation attributions, e.g. operation type is loading or
storing, cache level, it's snooping or remote accessing, etc.  At the
end, the 'data_src' will be parsed by perf mem/c2c tools to display
human readable strings.

This patch is to fill the 'data_src' field in the synthesized samples
base on different types.  Currently perf tool can display statistics for
L1/L2/L3 caches but it doesn't support the 'last level cache'.  To fit
to current implementation, 'data_src' field uses L3 cache for last level
cache.

Before this commit, perf mem report looks like this:
  # Samples: 75K of event 'l1d-miss'
  # Total weight : 75951
  # Sort order   : local_weight,mem,sym,dso,symbol_daddr,dso_daddr,snoop,tlb,locked
  #
  # Overhead  Samples  Local Weight  Memory access  Symbol                  Shared Object  Data Symbol             Data Object  Snoop  TLB access
  # ........  .......  ........