arm64/mm: Add soft-dirty page tracking support

Use the final remaining PTE SW bit (63) for soft-dirty tracking. The
standard handlers are implemented for set/test/clear for both pte and
pmd. Additionally we must also track the soft-dirty state as a pte swp
bit, so use a free swap entry pte bit (61).

There are a few complexities worth calling out:

  - The semantic of soft-dirty calls for having it auto-set by
    pte_mkdirty(). But the arch code would previously call pte_mkdirty()
    for various house-keeping operations such as gathering dirty bits
    into a pte across a contpte block. These operations must not cause
    soft-dirty to be set. So an internal version, __pte_mkdirty(), has
    been created that does not manipulate soft-dirty, and pte_mkdirty()
    is now a wrapper around that, which also sets the soft-dirty bit.

  - For a region with soft-dirty tracking enabled, it works by
    wrprotecting the ptes, causing a write to fault, where the handler
    calls pte_mkdirty(ptep_get()) (which causes soft-dirty to be set),
    then the resulting pte is written back with ptep_set_access_flags().
    So the arm64 version of ptep_set_access_flags() now needs to
    explicitly also set the soft-dirty bit to prevent loss.

The patch is very loosely based on a similar patch posted by Shivansh
Vij <shivanshvij@outlook.com>, at the below link.

Primary motivation for adding soft-dirty support is to allow
Checkpoint-Restore in Userspace (CRIU) to be able to track a memory
page's changes if we want to enable pre-dumping, which is important for
live migration.

Link: https://lore.kernel.org/linux-arm-kernel/MW4PR12MB687563EFB56373E8D55DDEABB92B2@MW4PR12MB6875.namprd12.prod.outlook.com/


Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>