net/9p: Handle get_user_pages_fast return properly

Use proper data type to handle get_user_pages_fast error condition. Also
do not treat EFAULT error as fatal.

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Signed-off-by: Venkateswararao Jujjuri <jvrao@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>