perf probe: Find fentry mcount fuzzed parameter location

perf probe: Find fentry mcount fuzzed parameter location

At this point, --fentry (mcount function entry) option for gcc fuzzes
the debuginfo variable locations by skipping the mcount instruction
offset (on x86, this is a 5 byte call instruction).

This makes variable searching fail at the entry of functions which
are mcount'ed.

e.g.)
Available variables at vfs_read
        @<vfs_read+0>
                (No matched variables)

This patch adds additional location search at the function entry point
to solve this issue, which tries to find the earliest address for the
variable location.

Note that this only works with function parameters (formal parameters)
because any local variables should not exist on the function entry
address (those are not initialized yet).

With this patch, perf probe shows correct parameters if possible;
 # perf probe --vars vfs_read
 Available variables at vfs_read
         @<vfs_read+0>
                 char*   buf
                 loff_t* pos
                 size_t  count
                 struct file*    file

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20131011071025.15557.13275.stgit@udc4-manage.rcp.hitachi.co.jp
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>