nfsd: Fix session slot freeing for NFSv4.1/4.2

Without this patch the NFSv4.1/4.2 server erroneously
always frees session slot zero for callbacks. This only
affects 4.1/4.2 mounts if the server has delegations
enabled or is a pNFS configuration. Even for those
cases, the effect is mainly to only use slot 0 for
callbacks, serializing all of them. There is a slight
chance that callbacks will fail if the client performs
them in a different order than received on the TCP
connection.

If this bug affects your server, you will see console
messages like:

newnfs_request: Bad session slot

This patch fixes the problem. Found during a recent
IETF NFSv4 testing event.

PR: 263728

(cherry picked from commit 271f6d52a606e86c11b366082f11fe69350f24da)