parisc: Drop WARN_ON_ONCE() from flush_cache_vmap

commit 4eab1c27 upstream.

I have observed warning to occassionally trigger.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v5.12+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>