parisc: Ensure page alignment in flush functions

Matthew Wilcox noticed, that if ARCH_HAS_FLUSH_ON_KUNMAP is defined
(which is the case for PA-RISC), __kunmap_local() calls
kunmap_flush_on_unmap(), which may call the parisc flush functions with
a non-page-aligned address and thus the page might not be fully flushed.

This patch ensures that flush_kernel_dcache_page_asm() and
flush_kernel_dcache_page_asm() will always operate on page-aligned
addresses.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # v6.0+