nfscl: Fix setup of Sequence when all slots marked bad

Commit 40ada74ee1da modified the NFSv4.1/4.2 client so
that it would issue a DestroySession to the server when
all session slots are marked bad. Once this is done,
the Sequence operation should get a NFSERR_BADSESSION
reply from the server.

Without this patch, the code was setting ND_HASSLOTID
when, in fact, there was no slot marked in use by
nfsv4_sequencelookup(). This would result in the
code freeing a slot not in use. The effect of this
was minimal, since the session was already destroyed.

This patch fixes the code so that it does not set
ND_HASSLOTID for this case.

MFC after: 2 weeks