fib: support "midchain delegate" removal

fib: support "midchain delegate" removal

Type: improvement

Currently, once an adjacency is stacked on a FIB entry via
adj_midchain_delegate_stack(), "midchain delegate" is created for the
adjacency and the FIB index is stored there. And all further calls to
adj_midchain_delegate_stack() even passing another FIB index will cause
the function to still use the stored one. In other words, there is
currently no way to stack an adjacency on another FIB index if "midchain
delegate" already exists for it.

Being able to stack on another FIB index is needed for the wireguard
plugin. As per the protocol, peers can roam between different external
endpoints. When an authenticated packet is received and it was sent from
a different endpoint than currently stored, the endpoint needs to be
updated and all futher communication needs to happen with that endpoint.
Thus, the corresponding to that peer adjacencies need to be stacked on
the FIB entry that corresponds to the new endpoint.

With this change, add adj_midchain_delegate_remove() that removes
"midchain delegate". When stacking on another FIB entry is needed,
existing "midchain delegate" can be removed and then, a new one created
with a new FIB index via adj_midchain_delegate_stack().

Signed-off-by: Alexander Chernavin <[email protected]>
Change-Id: Ibc1c99b248a5ef8ef64867f39f494fab627a1741