In bpf_setf(), protect against races between multiple user threads attempting to change the BPF filter on a BPF descriptor at the same time: retrieve the old filter pointer under the same locked region as setting the new pointer.
In bpf_setf(), protect against races between multiple user threads
attempting to change the BPF filter on a BPF descriptor at the same
time: retrieve the old filter pointer under the same locked region
as setting the new pointer.

MFC after:	3 days

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase