nfsd: fix kernel test robot warning in SSC code

Fix by initializing pointer nfsd4_ssc_umount_item with NULL instead of 0.
Replace return value of nfsd4_ssc_setup_dul with __be32 instead of int.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>