parisc: Drop strnlen_user() in favour of generic version

As suggested by Arnd Bergmann, drop the parisc version of
strnlen_user() and switch to the generic version.

Suggested-by: Arnd Bergmann <arnd@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Helge Deller <deller@gmx.de>