NFSD: Fix error return code in nfsd4_interssc_connect()

'status' has been overwritten to 0 after nfsd4_ssc_setup_dul(), this
cause 0 will be return in vfs_kern_mount() error case. Fix to return
nfserr_nodev in this error.

Fixes: f4e44b39 ("NFSD: delay unmount source's export after inter-server copy completed.")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>