perf augmented_syscalls: Check probe_read_str() return separately

Using a value returned from probe_read_str() to tell how many bytes to
copy using perf_event_output() has issues in some older kernels, like
4.17.17-100.fc27.x86_64, so separate the bounds checking done on how
many bytes to copy to a separate variable, so that the next patch has
only what is being done to make the test pass on older BPF validators.

For reference, see the discussion in this thread:

  https://www.spinics.net/lists/netdev/msg480099.html

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-jtsapwibyxrnv1xjfsgzp0fj@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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