sparc64: Fix kernel thread stack termination.

Because of the silly way I set up the initial stack for
new kernel threads, there is a loop at the top of the
stack.

To fix this, properly add another stack frame that is copied
from the parent and terminate it in the child by setting
the frame pointer in that frame to zero.

Signed-off-by: David S. Miller <davem@davemloft.net>