perf probe: Fix use of kernel image path given by 'k' option

Users were not being able to have the explicitely specified vmlinux
pathname used, instead a search on the vmlinux path was always being
made.

Reported-by: Francis Moreau <francis.moro@gmail.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: 2nddept-manager@sdl.hitachi.co.jp
Cc: Francis Moreau <francis.moro@gmail.com>
Cc: Franck Bui-Huu <vagabon.xyz@gmail.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
LPU-Reference: <m3hbelydz8.fsf_-_@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>