NFSD: Fix NFS server build errors

yuehaibing@huawei.com reports the following build errors arise when
CONFIG_NFSD_V4_2_INTER_SSC is set and the NFS client is not built
into the kernel:

fs/nfsd/nfs4proc.o: In function `nfsd4_do_copy':
nfs4proc.c:(.text+0x23b7): undefined reference to `nfs42_ssc_close'
fs/nfsd/nfs4proc.o: In function `nfsd4_copy':
nfs4proc.c:(.text+0x5d2a): undefined reference to `nfs_sb_deactive'
fs/nfsd/nfs4proc.o: In function `nfsd4_do_async_copy':
nfs4proc.c:(.text+0x61d5): undefined reference to `nfs42_ssc_open'
nfs4proc.c:(.text+0x6389): undefined reference to `nfs_sb_deactive'

The new inter-server copy code invokes client functions. Until the
NFS server has infrastructure to load the appropriate NFS client
modules to handle inter-server copy requests, let's constrain the
way this feature is built.

Reported-by: YueHaibing <yuehaibing@huawei.com>
Fixes: ce0887ac ("NFSD add nfs4 inter ssc to nfsd4_copy")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: YueHaibing <yuehaibing@huawei.com> # build-tested