lockd: rename lockd_create_svc() to lockd_get()

lockd_create_svc() already does an svc_get() if the service already
exists, so it is more like a "get" than a "create".

So:
 - Move the increment of nlmsvc_users into the function as well
 - rename to lockd_get().

It is now the inverse of lockd_put().

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>