bpf: Optimize lpm trie delete

Before the delete operator was added, this datastructure maintained
an invariant that intermediate nodes were only present when necessary
to build the tree.  This patch updates the delete operation to reinstate
that invariant by removing unnecessary intermediate nodes after a node is
removed and thus keeping the tree structure at a minimal size.

Suggested-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Craig Gallek <kraig@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>