perf lock: Redo __cmd_report

This function should be straightforward, and we can remove some trivial
logic by moving the functionality of read_events() into __cmd_report() -
thus allowing a new session to be properly deleted.

Since the 'info' subcommand also needs to process the recorded events,
add a 'display_info' flag to differentiate between report and info
commands.

Furthermore, this patch also calls perf_session__has_traces(), making
sure that we don't compare apples and oranges, fixing a segfault when
using an perf.data file generated by a different subcommand. ie:

./perf mem record sleep 1
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.017 MB perf.data (~724 samples) ]

./perf lock report
Segmentation fault (core dumped)

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1378693159-8747-5-git-send-email-davidlohr@hp.com


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