[RFC] minor kfence patches

[Sven Schnelle]

i'm currently looking into adding support for KFENCE to the s390
architecture. So far everything is straightforward, and i get the
kfence testsuite to pass, which is good! :)

One minor thing i encountered is that for a translation exception,
s390 only reports the page address, but not the complete address. I
worked around that by adding a function to kfence which allows to mask
out certain bits during unit testing. I wonder whether that should be a
weak function that can be implemented by architectures if required, some
kconfig option, or some other way?

The other thing is that s390 (and some other architectures) has different
address spaces for kernel and user space, so the decision whether an
address belongs to user or kernel space cannot be made by just looking
at the address. I added a small if (user_mode(regs)) check to
kfence_handle_page_fault(). But this could of also be done in the
architecture specific code.

What do you think?

Thanks,
Sven