sparc64: Use ftrace_graph_get_ret_stack() instead of curr_ret_stack

The structure of the ret_stack array on the task struct is going to
change, and accessing it directly via the curr_ret_stack index will no
longer give the ret_stack entry that holds the return address. To access
that, architectures must now use ftrace_graph_get_ret_stack() to get the
associated ret_stack that matches the saved return address.

Cc: sparclinux@vger.kernel.org
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>