perf augmented_raw_syscalls: Move the probe_read_str to a separate function

perf augmented_raw_syscalls: Move the probe_read_str to a separate function

One more step into copying multiple filenames to support syscalls like
rename*.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-xdqtjexdyp81oomm1rkzeifl@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>