netrom: Fix a data-race around sysctl_netrom_transport_requested_window_size

[ Upstream commit a2e706841488f474c06e9b33f71afc947fb3bf56 ]

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>