sparc64: Fix recursion in stack overflow detection handling.

sparc64: Fix recursion in stack overflow detection handling.

The calls down into prom_printf() when we detect an overflowed stack
can recurse again since the overflow stack will be "below" the current
kernel stack limit.

Prevent this by just returning straight if we are on the stack
overflow safe stack already.

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