ipv4: Fix NULL vs error pointer check in inet_blackhole_dev_init()

[ Upstream commit a51160f8da850a65afbf165f5bbac7ffb388bf74 ]

The inetdev_init() function never returns NULL.  Check for error
pointers instead.

Fixes: 22600596b675 ("ipv4: give an IPv4 dev to blackhole_netdev")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/aLaQWL9NguWmeM1i@stanley.mountain


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>