When we treclaim and trecheckpoint there's an unavoidable period when r1
will not be a valid kernel stack pointer.

This patch clears the MSR recoverable interrupt (RI) bit over these
regions to indicate we have an invalid kernel stack pointer.

For treclaim, the region over which we clear MSR RI is larger than
required to avoid the need for an extra costly mtmsrd.

Thanks to Paulus for suggesting this change.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>