net/9p: Make 9P2000.L the default protocol for 9p file system

If we dont' specify a protocol version default to 9P2000.L. 9P2000.L
have better support for posix semantic and is where all the recent development
is happening.

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