remove the last set_fs() in common code, and remove it for x86 and powerpc v2

[Christoph Hellwig]

Hi all,

this series removes the last set_fs() used to force a kernel address
space for the uaccess code in the kernel read/write/splice code, and then
stops implementing the address space overrides entirely for x86 and
powerpc.

The file system part has been posted a few times, and the read/write side
has been pretty much unchanced.  For splice this series drops the
conversion of the seq_file and sysctl code to the iter ops, and thus loses
the splice support for them.  The reasons for that is that it caused a lot
of churn for not much use - splice for these small files really isn't much
of a win, even if existing userspace uses it.  All callers I found do the
proper fallback, but if this turns out to be an issue the conversion can
be resurrected.

Besides x86 and powerpc I plan to eventually convert all other
architectures, although this will be a slow process, starting with the
easier ones once the infrastructure is merged.  The process to convert
architectures is roughtly:

 (1) ensure there is no set_fs(KERNEL_DS) left in arch specific code
 (2) implement __get_kernel_nofault and __put_kernel_nofault
 (3) remove the arch specific address limitation functionality

Changes since v1:
 - drop the patch to remove the non-iter ops for /dev/zero and
   /dev/null as they caused a performance regression
 - don't enable user access in __get_kernel on powerpc
 - xfail the set_fs() based lkdtm tests

Diffstat: