net/ipv6: Fix a test against 'ipv6_find_idev()' return value

[ Upstream commit 178fe94405bffbd1acd83b6ff3b40211185ae9c9 ]

'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 <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>