perf trace: Separate tp syscall field caching into init routine to be reused

We need to set this in evsels coming out of a perf.data file header, not
just for new ones created for live sessions.

So separate the code that caches the syscall entry/exit tracepoint
format fields into a new function that will be used in the next
changeset.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
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/r/20131112115700.GC4053@ghostprotocols.net


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>