netrom: Fix data-races around sysctl_net_busy_read

We need to protect the reader reading the sysctl value because the
value can be changed concurrently.

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>