netfilter: nf_log: fix error on write NONE to logger choice sysctl

It is hard to unbind nf-logger:

  echo NONE > /proc/sys/net/netfilter/nf_log/0
  bash: echo: write error: No such file or directory

  sysctl -w net.netfilter.nf_log.0=NONE
  sysctl: setting key "net.netfilter.nf_log.0": No such file or directory
  net.netfilter.nf_log.0 = NONE

You need explicitly send '\0', for instance like:

  echo -e "NONE\0" > /proc/sys/net/netfilter/nf_log/0

That seem to be strange, so fix it using proc_dostring.

Now it works fine:
   modprobe nfnetlink_log
   echo nfnetlink_log > /proc/sys/net/netfilter/nf_log/0
   cat /proc/sys/net/netfilter/nf_log/0
   nfnetlink_log
   echo NONE > /proc/sys/net/netfilter/nf_log/0
   cat /proc/sys/net/netfilter/nf_log/0
   NONE

v2: add missed error check for proc_dostring

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>