ARM: make_coherent(): fix problems with highpte, part 2

update_mmu_cache() is called with the page table for the faulted-in
page still mapped.  We need to modify the PTE for this page to ensure
coherency with other shared mappings when multiple shared mappings
exist within a MM.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>