parisc: Simplify and speed up get_user() and put_user()

This patch simplifies the code for get_user() and put_user() a lot.

Instead of accessing kernel memory (%sr0) and userspace memory (%sr3)
hard-coded in the assembler instruction, we now preload %sr2 with either
%sr0 (for accessing KERNEL_DS) or with sr3 (to access USER_DS) and
use %sr2 in the load directly.

The generated code avoids a branch and speeds up execution by generating
less assembler instructions.

Signed-off-by: Helge Deller <deller@gmx.de>
Tested-by: Rolf Eike Beer <eike-kernel@sf-tec.de>