From: Christophe JAILLET <redacted>
Date: Fri, 28 Dec 2018 23:28:21 +0100

'ipv6_find_idev()' returns NULL on error, not an error pointer.
Update the test accordingly and return -ENOBUFS, as already done in
'addrconf_add_dev()', if NULL is returned.

Fixes: ("ipv6: allow userspace to add IFA_F_OPTIMISTIC addresses")
Signed-off-by: Christophe JAILLET <redacted>

Applied and queued up for -stable.

Please provide the SHA1_ID in the Fixes: tag next time, I fixed it up
for you.