netrom: Fix data-races around sysctl_netrom_network_ttl_initialiser

[ Upstream commit 119cae5ea3f9e35cdada8e572cc067f072fa825a ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>