9p: explicitly deny setlease attempts

9p is a remote network protocol, and it doesn't support asynchronous
notifications from the server. Ensure that we don't hand out any leases
since we can't guarantee they'll be broken when a file's contents
change.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>