Check for required sample attributes using evsel rather than sample_type
in the session header. If the attribute for a default field is not
present for the event type (e.g., new command operating on file from
older kernel) the field is removed from the output list.

Expected event types must exist. For example, if a user specifies

  -f trace:time,trace -f sw:time,cpu,sym

the perf.data file must contain both tracepoints and software events
(ie., it is an error if either does not exist in the file).

Attribute checking is done once at the beginning of perf-script rather
than for each sample.

v1 -> v2:
- addressed comments from acme

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1302148460-570-1-git-send-email-daahern@cisco.comSigned-off-by: David Ahern <daahern@cisco.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>