NFSD: keep track of the number of v4 clients in the system

[ Upstream commit 0926c395 ]

Add counter nfs4_client_count to keep track of the total number
of v4 clients, including courtesy clients, in the system.

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