arm64: uaccess: Mask __user pointers for __arch_{clear, copy_*}_user

arm64: uaccess: Mask __user pointers for __arch_{clear, copy_*}_user

Rewritting from commit f71c2ffcb20d upstream. On LTS 4.9, there has no
raw_copy_from/to_user, neither __copy_user_flushcache, and it isn't good
idead to pick up them. The following is origin commit log, that's also
applicable for the new patch.

    Like we've done for get_user and put_user, ensure that user pointers
    are masked before invoking the underlying __arch_{clear,copy_*}_user
    operations.

    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Alex Shi <alex.shi@linaro.org>