net/sysctl: zero-init stable_secret strings

Sting field sysctl_entry.sarg has wrong length in protobuf encoding in
netdev-9 image, according to
https://developers.google.com/protocol-buffers/docs/encoding :

In netdev-9.img binary representation of sarg field is: "1a 2c 32 36 30
37 3a 66 30 64 30 3a 31 30 30 32 3a 30 30 35 31 3a 30 30 30 30 3a 30 30
30 30 3a 30 30 30 30 3a 30 30 30 34 0a c0 f4 a7 01"
Field key is 0x1a - means type 2(Length-delimited field), field 3 - that
is sarg field.
Field len is 0x2c which is 44, first 40-byte
"2607:f0d0:1002:0051:0000:0000:0000:0004\0" and then last 4 - "c0 f4 a7
01" where 0xc0 is not utf-8, and that makes crit fail.

In sysctl_op we just read() from sysctl so no '\0' is added in the end
of the string. So we can zero-init arrays and that will fix the issue.

https://github.com/xemul/criu/issues/161



*Alternatively or additionaly we can put '\0' in the end of string in
sysctl_read_char to make it harder to make such a mistake in future.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>