s390/ftrace: provide working ftrace_return_address()

The common code ftrace_return_address(n), which is just a wrapper for
__builtin_return_address(n), will only work for n > 0 if CONFIG_FRAME_POINTER
is set to 'y'. Otherwise it will return 0.
Since on s390 we will never have that config option set to 'y'
ftrace_return_address() won't work at all for n > 0.

Luckily we always compile the kernel with -mkernel-backchain which
in turn means that __builtin_return_address(n) will always work.

So let ftrace_return_address(n) map to __builtin_return_address(n).

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>