arm64/mm: Support huge pte-mapped pages in vmap

Implement the required arch functions to enable use of contpte in the
vmap when VM_ALLOW_HUGE_VMAP is specified. This speeds up vmap
operations due to only having to issue a DSB and ISB per contpte block
instead of per pte. But it also means that the TLB pressure reduces due
to only needing a single TLB entry for the whole contpte block.

Since vmap uses set_huge_pte_at() to set the contpte, that API is now
used for kernel mappings for the first time. Although in the vmap case
we never expect it to be called to modify a valid mapping so
clear_flush() should never be called, it's still wise to make it robust
for the kernel case, so amend the tlb flush function if the mm is for
kernel space.

Tested with vmalloc performance selftests:

  # kself/mm/test_vmalloc.sh \
	run_test_mask=1
	test_repeat_count=5
	nr_pages=256
	test_loop_count=100000
	use_huge=1

Duration reduced from 1274243 usec to 1083553 usec on Apple M2 for 15%
reduction in time taken.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>