pimd: Prevent vxlan from causing a S,G RPT Prune in some cases

pimd: Prevent vxlan from causing a S,G RPT Prune in some cases

Upon startup the pim vxlan code initiates a pim null register
send for the S,G and sends a *,G join towards the RP at the same
time.  Since a S,G upstream is created in the vxlan code with
the appropriate flags, the *,G join has the embedded S,G RPT
Prune.  When an intermediate route receives this *,G RPT Prune
it creates a blackhole S,G route since this particular intermediate
router has not received a join from the RP yet( say the packet is
lost, or that part of the network is slower coming up ).

Let's try to intelligently decide that the S,G RPT Prune
should not be sent as part of the *,G join until the actual
S,G join from the RP reaches this box.  Then we can make
intelligent decisions about whether or not to send it
out.

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