perf test vfs_getname: Disable ~/.perfconfig to get default output

To get the expected output we have to ignore whatever changes the user
has in its ~/.perfconfig file, so set PERF_CONFIG to /dev/null to
achieve that.

Before:

  # egrep 'trace|show_' ~/.perfconfig
  [trace]
  	show_zeros = yes
  	show_duration = no
  	show_timestamp = no
  	show_arg_names = no
  	show_prefix = yes
  # echo $PERF_CONFIG

  # perf test "trace + vfs_getname"
  70: Check open filename arg using perf trace + vfs_getname: FAILED!
  # export PERF_CONFIG=/dev/null
  # perf test "trace + vfs_getname"
  70: Check open filename arg using perf trace + vfs_getname: Ok
  #

After:

  # egrep 'trace|show_' ~/.perfconfig
  [trace]
  	show_zeros = yes
  	show_duration = no
  	show_timestamp = no
  	show_arg_names = no
  	show_prefix = yes
  # echo $PERF_CONFIG

  # perf test "trace + vfs_getname"
  70: Check open filename arg using perf trace + vfs_getname: Ok
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Link: https://lkml.kernel.org/n/tip-3up27pexg5i3exuzqrvt4m8u@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>