nd_integrity_init is only called from a single place.  Open code it
there, and use IS_ENABLED to remove the need for an extra stub.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240306142739.237234-2-hch@lst.deSigned-off-by: Ira Weiny <ira.weiny@intel.com>