NFSD: use per-operation statidx for callback procedures

The callback RPC procedure table uses NFSPROC4_CB_##call for
p_statidx, which maps CB_NULL to index 0 and every
compound-based callback (CB_RECALL, CB_LAYOUT, CB_OFFLOAD,
etc.) to index 1. All compound callback operations therefore
share a single statistics counter, making per-operation
accounting impossible.

Assign p_statidx from the NFSPROC4_CLNT_##proc enum instead,
giving each callback operation its own counter slot. The
counts array is already sized by ARRAY_SIZE(nfs4_cb_procedures),
so no allocation change is needed.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>