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 <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Luis Cláudio Gonçalves <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Taeung Song <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/shell/trace+probe_vfs_getname.sh

···
 	exit $err
 fi
 
+# Do not use whatever ~/.perfconfig file, it may change the output
+# via trace.{show_timestamp,show_prefix,etc}
+export PERF_CONFIG=/dev/null
+
 trace_open_vfs_getname
 err=$?
 rm -f ${file}