fs/9p: translate O_TRUNC into OTRUNC

[ Upstream commit 87de39e7 ]

This one hits both 9P2000 and .u as it appears v9fs has never translated
the O_TRUNC flag.

Signed-off-by: Joakim Sindholt <opensource@zhasha.com>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>