NFSD: Implement CB_SEQUENCE referring call lists

The slot index number of the current COMPOUND has, until now, not
been needed outside of nfsd4_sequence(). But to record the tuple
that represents a referring call, the slot number will be needed
when processing subsequent operations in the COMPOUND.

Refactor the code that allocates a new struct nfsd4_slot to ensure
that the new sl_index field is always correctly initialized.

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