[POWERPC] Fix switch_slb handling of 1T ESID values

Now that we have 1TB segment size support, we need to be using the
GET_ESID_1T macro when comparing ESID values for pc, stack, and
unmapped_base within switch_slb().   A new helper function called
esids_match() contains the logic for deciding when to call GET_ESID
and GET_ESID_1T.

This fixes a duplicate-slb-entry inspired machine-check exception I
was seeing when trying to run java on a power6 partition.

Tested on power6 and power5.

Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>