[SPARC64]: Fix bogus flush instruction usage.

[SPARC64]: Fix bogus flush instruction usage.

Some of the trap code was still assuming that alternate
global %g6 was hard coded with current_thread_info().
Let's just consistently flush at KERNBASE when we need
a pipeline synchronization.  That's locked into the TLB
and will always work.

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