In Linux 4.14 and 4.19 these architectures still have their own
implementations of get_user_pages_fast(). These also need to force
the write flag on when taking the fast path.
Fixes: 407faed92b4a ("gup: document and work around "COW can break either way" issue")
Fixes: 5e24029791e8 ("gup: document and work around "COW can break either way" issue")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>