perf trace: Show path associated with fd in live sessions

For live sessions we can just access /proc to map an fd to its path, on
a best effort way, i.e. sometimes the fd will have gone away when we try
to do the mapping, as it is done in a lazy way, only when a reference to
such fd is made then the path will be looked up in /proc.

This is disabled when processing perf.data files, where we will have to
have a way to get getname events, be it via an on-the-fly 'perf probe'
event or after a vfs_getname tracepoint is added to the kernel.

A first step will be to synthesize such event for the use cases where
the threads in the monitored workload exist already.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-1r1ti33ye1666jezu2d8q1c3@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>