[parisc-linux] space registers

[Matthew Wilcox]

a small optimisation just occurred to me.

right now, when we switch to kernel space, we set all of sr4-sr7 to 0
(for the kernel mapping).  we don't need to do that since the kernel is
entirely in sr7's domain.  this has the added bonus that badly written
drivers which blindly dereference userspace pointers will work on
parisc as well as x86.  we can also lazily restore sr4-6 on exit from
kernel space if we're switching back to the same task which called us.
this optimisation may not be worthwhile, but i think setting sr4-6
on entry to the kernel is unnecessary.

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