Changes from v2:
 - number of slots is increased more quickly.  Every time the highest-number
   slot is used, we increase by at least 20%.
 - when xa_store() is used in a context where we no that no allocation can
   happen, pass '0' as the GFP flag.
 - report target slots as well as total slots in /proc/fs/nfsd/clients/*/info

I've stay with reporting session information in the client info file
rather than creating a directory or using netlink.  I think the client
info file is simple and adequate.

I still haven't added support for CB_RECALL_SLOT.  While it is helpful,
it isn't essential.  I'll probably try to add it after the current
series lands.

Thanks,
NeilBrown