[SPARC64]: Do not write garbage into %pstate in tsb_context_switch().

For SUN4V, we were clobbering %o5 to do the hypervisor call.
This clobbers the saved %pstate value and we end up writing
garbage into that register as a result.  Oops.

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