parisc: Fix flush_anon_page on PA8800/PA8900

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 <[email protected]>
Signed-off-by: Helge Deller <[email protected]>
Cc: [email protected]   # v5.18+