The 'type' sort key is to aggregate hist entries by data type they
access.  Add mem_type field to hist_entry struct to save the type.  If
hist_entry__get_data_type() returns NULL, it'd use the 'unknown_type'
instance.

Committer testing:

Before:

  # perf mem record  sleep 2s
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.037 MB perf.data (4 samples) ]
  root@number:/home/acme/Downloads# perf report --stdio -s type
  Error:
  Unknown --sort key: `type'
   Usage: perf report [<options>]

      -s, --sort <key[,key2...]>
                            sort by key(s): overhead overhead_sys overhead_us overhead_guest_sys
                            overhead_guest_us overhead_children sample period
                            pid comm dso symbol parent cpu socket srcline srcfile
                            local_weight weight transaction trace symbol_size
                            dso_size cgroup cgroup_id ipc_null time code_page_size
                            local_ins_lat ins_lat local_p_stage_cyc p_stage_cyc
                            addr local_retire_lat retire_lat simd dso_from dso_to
                            symbol_from symbol_to mispredict abort in_tx cycles
                            srcline_from srcline_to ipc_lbr addr_from addr_to
                            symbol_daddr dso_daddr locked tlb mem snoop dcacheline
                            symbol_iaddr phys_daddr data_page_size blocked
  #

After:

  # perf report --stdio -s type
  # To display the perf.data header info, please use --header/--header-only options.
  #
  #
  # Total Lost Samples: 0
  #
  # Samples: 4  of event 'cpu_atom/mem-loads,ldlat=30/P'
  # Event count (approx.): 7
  #
  # Overhead  Data Type
  # ........  .........
  #
     100.00%  (unknown)

  #
  # (Tip: Print event counts in CSV format with: perf stat -x,)
  #
  # rpm -q kernel-debuginfo
  kernel-debuginfo-6.6.4-200.fc39.x86_64
  # uname -r
  6.6.4-200.fc39.x86_64
  #
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: linux-toolchains@vger.kernel.org>
Cc: linux-trace-devel@vger.kernel.org>
Link: https://lore.kernel.org/r/20231213001323.718046-9-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>