fs/9p: Pass the correct user credentials during attach

We need to make sure we pass the right uid value
during attach. dotl is similar to dotu in this regard.
Without this mapped security model on dotl doesn't work

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>