parisc: fixes and cleanups in page cache flushing (4/4)

CONFIG_PARISC_TMPALIAS enables clear_user_highpage and copy_user_highpage.
These are essentially alternative implementations of clear_user_page and
copy_user_page.  They don't have anything to do with x86 high pages, but they
build on the infrastructure to save a few instructions.  Read the comment in
clear_user_highpage as it is very important to the implementation.  For this
reason, there isn't any gain in using the TMPALIAS/highpage approach.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>