NFSD: Rename the fields in copy_stateid_t

[ Upstream commit 781fde1a ]

Code maintenance: The name of the copy_stateid_t::sc_count field
collides with the sc_count field in struct nfs4_stid, making the
latter difficult to grep for when auditing stateid reference
counting.

No behavior change expected.

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