net/9p: Return error if we fail to encode protocol data

We need to return error in case we fail to encode data in protocol buffer.
This patch also return error in case of a failed copy_from_user.

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