9p: do not crash on unknown lock status code

Current 9p implementation will crash whole system if sees unknown lock
status code. It's trivial target for DOS: 9p server can produce such
code easily.

Let's fallback more gracefully: warning in dmesg + -ENOLCK.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>