fs/9p: Fix bit operation logic error

This re-introduces a fix that somehow got dropped during rebase of the
current series in for-next.  When writeback is enabled, opens
are forced to support both read and write operations but with the
logic error other flags may be dropped unintentionaly.

Reported-by: Christophe Jaillet <christophe.jaillet@wanadoo.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>