pimd: register local VTEP-IP for each BUM MDT via NULL registers

pimd: register local VTEP-IP for each BUM MDT via NULL registers

For multicast vxlan tunnels we register the local VTEP-IP independent
of the prescence of BUM traffic i.e. we prime the pump. This
is acheived via NULL registers.

VxLAN orig entries with upstream in a PIM_REG_JOIN state are linked to
a work list for periodic NULL register transmission. Once the SPT setup
is complete the upstream-entry moves to a PIM_REG_PRUNE state and is
remved from the VxLAN work list.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>