[SPARC64]: Fix build failure when CONFIG_BUG is disabled.

When CONFIG_BUG is turned off, the standard trick of:

	switch (x) {
	case X:
	...
	case Y:
	...
	default:
		BUG();
	};

to mark impossible cases does not work because BUG() evalutes
to nothing and thus GCC just sees a fallthrough code path.

Add an explicit KERN_ERR log message and a do_exit() to trap
this case.

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