zebra: Fetch actual SRv6 encap src addr on startup

zebra: Fetch actual SRv6 encap src addr on startup

When zebra is started, someone may have configured an SRv6 encap source
address different from the default address ( :: ) in the kernel.

On startup, zebra should not assume that the actual SRv6 encap source
address is the default address ( :: ), but should retrieve the actual
source address from the kernel and put it in zebra configuration. In
other words, on startup we expect the actual SRv6 encap source
address and the configured one to be the same.

This commit makes the necessary changes to support the above.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>