RPC: Clean up RPC task structure

 Shrink the RPC task structure. Instead of storing separate pointers
 for task->tk_exit and task->tk_release, put them in a structure.

 Also pass the user data pointer as a parameter instead of passing it via
 task->tk_calldata. This enables us to nest callbacks.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>