Silence warnings if NFS service isn't enabled.

At boot ix-nfsd always runs, because it has to reload
the exports it calls service mountd reload.  If NFS
isn't enabled this will cause a warning to be printed to
the console.

We still run the reload unconditionally, this change just
supresses the warning if the NFS service isn't enabled.

A slightly less hacky fix might be to only run ix-nfsd if NFS
is enabled but that would not be a one-line fix, and there's
a bigger refactoring in progress to deal with NFS startup,
so this will do for now.