ipv6: propagate ipv6_add_dev's error returns out of ipv6_find_idev

Currently, ipv6_find_idev returns NULL when ipv6_add_dev fails,
ignoring the specific error value. This results in addrconf_add_dev
returning ENOBUFS in all cases, which is unfortunate in cases such as:

    # ip link add dummyX type dummy
    # ip link set dummyX mtu 1200 up
    # ip addr add 2000::/64 dev dummyX
    RTNETLINK answers: No buffer space available

Commit a317a2f1

 ("ipv6: fail early when creating netdev named all
or default") introduced error returns in ipv6_add_dev. Before that,
that function would simply return NULL for all failures.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>