sparc64: Use cpu_pgsz_mask for linear kernel mapping config.

This required a little bit of reordering of how we set up the memory
management early on.

We now only know the final values of kern_linear_pte_xor[] after we
take over the trap table and start processing TLB misses ourselves.

So once we fill those values in we re-clear the kernel's 4M TSB and
flush the TLBs.  That way if we find we support larger than 4M pages
we won't have any stale smaller page size entries in the TSB.

SUN4U Panther support for larger page sizes should now be extremely
trivial but I have no hardware on which to test it and I believe
that some of the sun4u TLB miss assembler needs to be audited first
to make sure it really can handle larger than 4M PTEs properly.

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