perf probe: Fix mis-estimation for shortening filename

Fix mis-estimation size for making a short filename.
Since the buffer size is 32 bytes and there are '@' prefix and
'\0' termination, maximum shorten filename length should be
30. This means, before searching '/', it should be 31 bytes.

LKML-Reference: <20100414223935.14630.11954.stgit@localhost6.localdomain6>
Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>