fib: Use the same adjacency that BFD is using

fib: Use the same adjacency that BFD is using

Type: improvement

When the adj subsystem is notified of a BFD session, it attempts to find the appropriate adjacency from the session's key.
This could lead to a mismatch between the adj used by BFD and that of FIB. The BFD session stores the adj it is using, so FIB uses that instead.
Since adj is now using the same adj as BFD, it does not need to maintain its own locks.
In BFD it is necessary to initialise the adj index used in INVALID and ensure it is not unlock before listeners are notified of the session delete.

Signed-off-by: Neale Ranns <neale@graphiant.com>
Change-Id: I9630867b10bb18969475299a0c754942a8df0f44