perf namespaces: Avoid get_current_dir_name dependency

perf namespaces: Avoid get_current_dir_name dependency

get_current_dir_name is a GNU extension not supported on, for example,
Android. There is only one use of it so let's just switch to getcwd to
avoid build and other complexity.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>