[SPARC64]: Fix unaligned access winfxup handling on SUN4V.

Another case where we have to force ourselves into global register
level one.  Also make sure the arguments passed to sun4v_do_mna() are
correct.

This area actually needs some more work, for example spill fixup is
not necessarily going to do the right thing for this case.

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