rpc: add target field to new upcall

This patch extends the new upcall by adding a "target" field
communicating who we want to authenticate to (equivalently, the service
principal that we want to acquire a ticket for).

Signed-off: Olga Kornievskaia <aglo@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>