That is a silly idea, many BSD developers specifically choose to go with BSD because they don't like the GPL license.
It's probably easier for Linux developers to take code from BSD.
And even that doesn't happen as often as you'd expect.
I wouldn't be surprised if in the Linux kernel there are more "shared development" drivers with FreeBSD (a driver that has a BSD-license or dual license and development on on the same code happend by Linux developers in the Linux kernel and FreeBSD developers on FreeBSD) than BSD-code that was copied.