LockD: service creation function introduced

This function creates service if it doesn't exist, or increases usage
counter if it does, and returns a pointer to it.  The usage counter will
be droppepd by svc_destroy() later in lockd_up().

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>