busybox: sysctl: ignore EIO of stable_secret below /proc/sys/net/ipv6/conf

Avoid confusing messages caused by EIO on reading
/proc/sys/net/ipv6/conf/*/stable_secret if those are not set yet.
Make it behave the same as procps(>=3.3.13).

Fixes:
$ sysctl -a | grep ipv6.conf | grep stable_secret
sysctl: error reading key 'net.ipv6.conf.all.stable_secret': Input/output error
sysctl: error reading key 'net.ipv6.conf.default.stable_secret': Input/output error
sysctl: error reading key 'net.ipv6.conf.eth0.stable_secret': Input/output error
sysctl: error reading key 'net.ipv6.conf.lo.stable_secret': Input/output error

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>