net: nfs: Only link in NFS code outside of SPL builds

While we have networking use cases within SPL we do not support loading
files via NFS at this point in time.  Disable calling nfs_start() so
that the NFS related code can be garbage collected at link time.

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>