NFSD: Fix trace_nfsd_slot_seqid_sequence

While running down the problem triggered by disconnect injection,
I noticed the "in use" string was actually never hooked up in this
trace point, so it always showed the traced slot as not in use. But
what might be more useful is showing all the slot status flags.

Also, this trace point can record and report the slot's index
number, which among other things is useful for troubleshooting slot
table expansion and contraction.

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