sparc64: Fix DAX handling via userspace access from kernel.

If we do a userspace access from kernel mode, and get a
data access exception, we need to check the exception
table just like a normal fault does.

The spitfire DAX handler was doing this, but such logic
was missing from the sun4v DAX code.

Reported-by: Dennis Gilmore <dgilmore@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>