Impact: fix failure of dynamic function tracer selftest

In a course of development, a developer does several makes on their
kernel. Sometimes, the make might do something abnormal. In the
case of running the recordmcount.pl script on an object twice,
the script will duplicate all the calls to mcount in the __mcount_loc
section.

On boot up, the dynamic function tracer is careful when it modifies
code, and performs several consistency checks. One is to not modify
the call site if it is not what it expects it to be. If a function
call site is listed twice, the first entry will convert the site
to a nop, and the second will fail because it expected to see a
call to mcount, but instead it sees a nop. Thus, the function tracer
is disabled.

Eric Sesterhenn reported seeing:

[    1.055440] ftrace: converting mcount calls to 0f 1f 44 00 00
[    1.055568] ftrace: allocating 29418 entries in 116 pages
[    1.061000] ------------[ cut here ]------------
[    1.061000] WARNING: at kernel/trace/ftrace.c:441

 [...]

[    1.060000] ---[ end trace 4eaa2a86a8e2da23 ]---
[    1.060000] ftrace failed to modify [<c0118072>] check_corruption+0x3/0x2d
[    1.060000]  actual: 0f:1f:44:00:00

This warning shows that check_corruption+0x3 already had a nop in
its place (0x0f1f440000). After compiling another kernel the problem
went away.

Later Eric Paris notice the same type of issue. Luckily, he saved
the vmlinux file that caused it. In the file we found a bunch of
duplicate mcount call site records, which lead us to the script.

Perhaps this problem only happens to people named Eric.

This patch changes the script to test if the __mcount_loc already
exists in the object file, and if it does, it will print out
an error message and kill the compile.

Reported-by: Eric Sesterhenn <snakebyte@gmx.de>
Reported-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>