BGP: Fix linkage between BGP instance and VRF structure

BGP: Fix linkage between BGP instance and VRF structure

The issue here has to do with the fact that VRFs (like interfaces) are not
actually getting deleted when they are removed - they remain present. This
leads to situations in which BGP may try to unlink more than once, which
messes up the reference count (lock) in the BGP instance.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-9419
Reviewed By: CCR-4302
Testing Done: Manual, also verified by Atul

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