s390/unwind: fix fgraph return address recovery

When HAVE_FUNCTION_GRAPH_RET_ADDR_PTR is defined, the return
address to the fgraph caller is recovered by tagging it along with the
stack pointer of ftrace stack. This makes the stack unwinding more
reliable.

When the fgraph return address is modified to return_to_handler,
ftrace_graph_ret_addr tries to restore it to the original
value using tagged stack pointer.

Fix this by passing tagged sp to ftrace_graph_ret_addr.

Fixes: d81675b6

 ("s390/unwind: recover kretprobe modified return address in stacktrace")
Cc: <stable@vger.kernel.org> # 5.18
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>