perf test shell: Use a fallback to get the pathname in vfs_getname

Some kernels, like 4.19.13-300.fc29.x86_64 in fedora 29, fail with the
existing probe definition asking for the contents of result->name,
working when we ask for the 'filename' variable instead, so add a
fallback to that.

Now those tests are back working on fedora 29 systems with that kernel:

  # perf test vfs_getname
  65: Use vfs_getname probe to get syscall args filenames   : Ok
  66: Add vfs_getname probe to get syscall args filenames   : Ok
  67: Check open filename arg using perf trace + vfs_getname: Ok
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-klt3n0i58dfqttveti09q3fi@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>