perf top: Don't stop if no kernel symtab is found

perf top: Don't stop if no kernel symtab is found

We now just warn the user about the fact and go on providing just
userspace samples.

This fixes a problem when no vmlinux is explicetely passed by the user,
thus symbol_conf.vmlinux_name is NULL, no suitable vmlinux is found, and
then we get:

 aldebaran:~> perf top -p 7557
 [kernel.kallsyms] with build id 44d9a989eabbd79e486bc079d6b743d397c204e0
 not found, continuing without symbols
 The (null) file can't be used

Reported-by: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.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>
Cc: Tom Zanussi <tzanussi@gmail.com>
Link: http://lkml.kernel.org/n/tip-cj2g81hn64wv2bipmqk4fy2m@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>