net/sysctl: silence read error in case of CTL_FLAGS_READ_EIO_SKIP

Because stable_secret sysctl c/r produces a lot of spam error messages
in tests: https://github.com/xemul/criu/issues/160

 But EIO for these
sysctl is valid behavior and no error need to be printed.

More general solution will also silence error messages for
sysctl_read_{u32,s32,u64}, but the flag CTL_FLAGS_READ_EIO_SKIP is
now used only for __CTL_STR and only for stable_secret so we can do
silence error only in sysctl_read_char.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Tested-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>