parisc: Fix flush_anon_page on PA8800/PA8900

Anonymous pages are allocated with the shared mappings colouring,
SHM_COLOUR. Since the alias boundary on machines with PA8800 and
PA8900 processors is unknown, flush_user_cache_page() might not
flush all mappings of a shared anonymous page. Flushing the whole
data cache flushes all mappings.

This won't fix all coherency issues with shared mappings but it
seems to work well in practice.  I haven't seen any random memory
faults in almost a month on a rp3440 running as a debian buildd
machine.

There is a small preformance hit.
Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org   # v5.18+

arch/parisc/kernel/cache.c

@@ -722,7 +722,10 @@ void flush_anon_page(struct vm_area_struct *vma, struct page *page, unsigned lon
 		return;
 
 	if (parisc_requires_coherency()) {
-		flush_user_cache_page(vma, vmaddr);
+		if (vma->vm_flags & VM_SHARED)
+			flush_data_cache();
+		else
+			flush_user_cache_page(vma, vmaddr);
 		return;
 	}