perf tools: Make vmlinux short name more like kallsyms short name

The previous patch changed kernel dso name from '[kernel.kallsyms]' to
vmlinux.  However it might add confusion to old users accustomed to the
old name.  So change the short name to '[kernel.vmlinux]' to reduce such
confusion.

Before:
  # Overhead  Command         Shared Object            Symbol
  # ........  ..............  .......................  ...............................
  #
       9.83%  swapper         vmlinux                  [k] intel_idle
       4.10%  awk             libc-2.20.so             [.] __strcmp_sse2
       1.86%  sed             libc-2.20.so             [.] __strcmp_sse2
       1.78%  netctl-auto     libc-2.20.so             [.] __strcmp_sse2
       1.23%  netctl-auto     libc-2.20.so             [.] __mbrtowc
       1.21%  firefox         libxul.so                [.] 0x00000000024b62bd
       1.20%  swapper         vmlinux                  [k] cpuidle_enter_state
       1.03%  sleep           vmlinux                  [k] copy_user_generic_unrolled

After:
  # Overhead  Command         Shared Object            Symbol
  # ........  ..............  .......................  ...............................
  #
       9.83%  swapper         [kernel.vmlinux]         [k] intel_idle
       4.10%  awk             libc-2.20.so             [.] __strcmp_sse2
       1.86%  sed             libc-2.20.so             [.] __strcmp_sse2
       1.78%  netctl-auto     libc-2.20.so             [.] __strcmp_sse2
       1.23%  netctl-auto     libc-2.20.so             [.] __mbrtowc
       1.21%  firefox         libxul.so                [.] 0x00000000024b62bd
       1.20%  swapper         [kernel.vmlinux]         [k] cpuidle_enter_state
       1.03%  sleep           [kernel.vmlinux]         [k] copy_user_generic_unrolled

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1415063674-17206-9-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>