NFSD: Fix sparse warning in nfssvc.c

fs/nfsd/nfssvc.c:36:6: warning: symbol 'inter_copy_offload_enable' was not declared. Should it be static?

The parameter was added by commit ce0887ac ("NFSD add nfs4 inter
ssc to nfsd4_copy"). Relocate it into the source file that uses it,
and make it static. This approach is similar to the
nfs4_disable_idmapping, cltrack_prog, and cltrack_legacy_disable
module parameters.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>