net: ipv6: Fix neighbor registration based on received RA message

When Router Advertisement with Source Link-Layer Address option is
received, host should register a new neighbor marked as STALE
(RFC 4861, ch. 6.3.4). This behavior was broken however, because
we always added a new neighbor in INCOMPLETE state before processing
SLLA option. In result, the entry was not updated to the STALE state,
and a redundant Neighbor Solicitation was sent.

Fix this by moving the code responsible for adding neighbor in
INCOMPLETE state after options processing, and only as a fallback
behavior if the SLLA option was not present.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>