pimd: Fix upstream state machine.

pimd: Fix upstream state machine.

RFC 4601 4.5.7:
Joined
      The downstream state machines and local membership information
      indicate that the router should join the shortest-path tree for
      this (S,G).

The code has a upstream state that is in 'J' state already
due to a S,G multicast packet received on an incoming
interface.  This packet has been forwarded to the RP.
The RP sees this and immediately sends a join towards
the S,G.

The code as originally written assumed that you could
not transition from a J state to a J state.  This is not
true.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>