set_fs removal for s390

[Christoph Hellwig <hch@lst.de>]

Hi s390 maintainers,

the base work to remove address space overrides using set_fs() has
landed in linux-next through this tree:

https://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git/log/?h=base.set_fs

and in addition to x86 and powerpc converted there, we have arm, arm64
and RISC-V conversion in progress.  That means s390 is the only "tier 1"
architecture still missing.  Due to the special instructions for
accessing the user space address space I don't really feel like doing
the conversion myself, but it would be great if we could get s390 done
for 5.10 as well.

The rought TODO list is:
 (1) actually stop using set_fs in the zrcypt driver.  I could prepare
     a crude untested patch for that if it helps.
 (2) implement the __get_user_nofault and __put_user_nofault helpers to
     safely access kernel memory.  These should behave like
     get_user/put_user under set_fs(KERNEL_DS), but not actually
     manipulate any task state to do so
 (3) unselect CONFIG_SET_FS and remove all the code related to overriding
     the address space(3) unselect CONFIG_SET_FS and remove all the code
     related to overriding the address space limit