dump/net/ipv6: net device and def/all configuration
dump/net/ipv6: net device and def/all configuration

* old MAX_CONF_OPT_PATH should be enough for ipv6 path array as
len('net/ipv6/conf//mldv2_unsolicited_report_interval') is 48
but len and size are almost back to back, so increase it a bit,
to don't worry about it.

reading stable_secret before it is initialized fails with EIO,
we can safely skip dumping it in that case as in new netns it
will be uninitialized by default

v6: use __CTL_STR without len in image type
v9: set flag CTL_FLAGS_READ_EIO_SKIP for stable_secret dumping

Signed-off-by: Pavel Tikhomirov <3e8c75167f9f00ca74287a4609f1507a2696e07e@virtuozzo.com>
Reviewed-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

efiop/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,efiop/criu,eabatalov/criu,efiop/criu,eabatalov/criu,KKoukiou/criu-remote,eabatalov/criu,KKoukiou/criu-remote,efiop/criu,efiop/criu,efiop/criu,eabatalov/criu,eabatalov/criu,KKoukiou/criu-remote,eabatalov/criu,KKoukiou/criu-remote