xen: arm: avoid reusing incorrect mappings when walking the p2m.

xen: arm: avoid reusing incorrect mappings when walking the p2m.

When we change which PT page we are mapping at a given level then we need to
invalidate any cached mappings further down the tree, otherwise we risk using
them because their offset might match but be based on a different offset
further up the table.

e.g. when remapping first then cur_first_offset and cur_second_offset (which
indicate the currently mapped second and third tables respectively) both become
invalid

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>