addrconf: always initialize sysctl table data

addrconf: always initialize sysctl table data

When sysctl performs restrict writes, it allows to write from
a middle position of a sysctl file, which requires us to initialize
the table data before calling proc_dostring() for the write case.

Fixes: 3d1bec99320d ("ipv6: introduce secret_stable to ipv6_devconf")
Reported-by: Sasha Levin <[email protected]>
Acked-by: Hannes Frederic Sowa <[email protected]>
Tested-by: Sasha Levin <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>