ARM: pgtable: consolidate set_pte_ext(TOP_PTE,...) + tlb flush

A number of places establish a PTE in our top page table and
immediately flush the TLB.  Rather than having this at every callsite,
provide an inline function for this purpose.

This changes some global tlb flushes to be local; each time we setup
one of these mappings, we always do it with preemption disabled which
would prevent us migrating to another CPU.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>