arm64: uaccess: implement unsafe accessors

arm64: uaccess: implement unsafe accessors

Current implementation of get/put_user_unsafe default to get/put_user
which toggle PAN before each access, despite having been told by the caller
that multiple accesses to user memory were about to happen.

Provide implementations for user_access_begin/end to turn PAN off/on and
implement unsafe accessors that assume PAN was already turned off.

Tested-by: Will Deacon <[email protected]>
Signed-off-by: Julien Thierry <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>