Add warning printf w.r.t. removal of sys/nfs/nfs_lock.c.

The code in sys/nfs/nfs_lock.c has not been run by default since March 2008
when it was replaced by the in kernel sys/nlm code.
It uses Giant, so it needs to be removed before the FreeBSD 13 release.
This will happen in a couple of months, since few if any users run
the code anyhow and can easily switch to the default in kernel NFSLOCKD.