nfscl: Re-initialize session seq#s when server shrinks slot table

When a NFSv4.1/4.2 server reduces the size of the slot table
for a session as indicated by a smaller value for sr_target_highest_slot
in a Sequence reply, the sequence numbers for the slots no
longer in use must be re-initialized. This is needed since the
slot table may be grown again by the server later.

The RFC did not make the need for the sequence numbers to be
re-initialized when a shrink/grow of the slot table size happens,
but this has now been confirmed as correct behaviour.

The patch adds the code that does this re-initialization.

I am not currently aware of a NFSv4.1/4.2 server where the
session slots fail if this is not done, but there may be such
a case.

MFC after: 2 weeks