nfscl: Add support for CB_RECALL_SLOT
The callback CB_RECALL_SLOT is required for NFSv4.1/4.2.
Fortunately, there does not appear to be any extant
NFSv4.1/4.2 servers that use it. Since commit b97a478896e9
fixed handling of session slot shrinking, this patch
adds support for CB_RECALL_SLOT, which shrinks the
number of session slots as well.
MFC after: 2 weeks