clean up and streamline probe_kernel_* and friends

[Christoph Hellwig]

Hi all,

this series start cleaning up the safe kernel and user memory probing
helpers in mm/maccess.c, and then allows architectures to implement
the kernel probing without overriding the address space limit and
temporarily allowing access to user memory.  It then switches x86
over to this new mechanism by reusing the unsafe_* uaccess logic.