perf annotate: Fix annotate context lines regression

The live annotation done in 'perf top' needs to limit the context before
lines that aren't filtered out by the min percent filter, if we don't do
that, the screen in a tty often is not enough for showing what is
interesting: lines with hits and a few source code lines before it.

Reported-by: Mike Galbraith <efault@gmx.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
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>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>