parisc: Switch user access functions to signal errors in r29 instead of r8

Use register r29 instead of register r8 to signal faults when accessing
user memory. In case of faults, the fixup routine will store -EFAULT in
this register.

This change saves up to 752 bytes on a 32bit kernel, partly because the
compiler doesn't need to save and restore the old r8 value on the stack.

bloat-o-meter results for usage with r29 register:
add/remove: 0/0 grow/shrink: 23/86 up/down: 228/-980 (-752)

bloat-o-meter results for usage with r28 register:
add/remove: 0/0 grow/shrink: 28/83 up/down: 296/-956 (-660)

Signed-off-by: Helge Deller <deller@gmx.de>