Before this patch we would not find a vmlinux, then try to pass
objdump "[kernel.kallsyms]" as the filename, it would get
confused and produce no output:

 [root@doppio ~]# perf annotate n_tty_write

 ------------------------------------------------
  Percent |      Source code & Disassembly of [kernel.kallsyms]
 ------------------------------------------------

Now we check that and emit meaningful warning:

 [root@doppio ~]# perf annotate n_tty_write
 Can't annotate n_tty_write: No vmlinux file was found in the
 path: [0] vmlinux
 [1] /boot/vmlinux
 [2] /boot/vmlinux-2.6.34-rc1-tip+
 [3] /lib/modules/2.6.34-rc1-tip+/build/vmlinux
 [4] /usr/lib/debug/lib/modules/2.6.34-rc1-tip+/vmlinux
 [root@doppio ~]#

This bug was introduced when we added automatic search for
vmlinux, before that time the user had to specify a vmlinux
file.

v2: Print the warning just for the first symbol found when no
    symbol name is specified, otherwise it will spam the screen
    repeating the warning for each symbol.
Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: <stable@kernel.org>
LKML-Reference: <1268669073-6856-1-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>