rtnetlink: unlock on error path in netlink_dump()

In c7ac8679 "rtnetlink: Compute and store minimum ifinfo dump
size", we moved the allocation under the lock so we need to unlock
on error path.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>