nfsd: fix kernel test robot warning in SSC code

[ Upstream commit f47dc2d3 ]

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>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>