arm64: mm: specialise pagetable allocators

We pass a size parameter to early_alloc and late_alloc, but these are
only ever used to allocate single pages. In late_alloc we always
allocate a single page.

Both allocators provide us with zeroed pages (such that all entries are
invalid), but we have no barriers between allocating a page and adding
that page to existing (live) tables. A concurrent page table walk may
see stale data, leading to a number of issues.

This patch specialises the two allocators for page tables. The size
parameter is removed and the necessary dsb(ishst) is folded into each.
To make it clear that the functions are intended for use for page table
allocation, they are renamed to {early,late}_pgtable_alloc, with the
related function pointed renamed to pgtable_alloc.

As the dsb(ishst) is now in the allocator, the existing barrier for the
zero page is redundant and thus is removed. The previously missing
include of barrier.h is added.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Cc: Laura Abbott <labbott@fedoraproject.org>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>