perf probe: Clean up redundant tests in show_line_range()

It also removes some superflous parentheses.

Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
LKML-Reference: <1292854685-8230-4-git-send-email-fbuihuu@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>