SUNRPC: cleanup GSS pipes usage

Currently gss auth holds RPC inode pointer which is now redundant since it
requires only pipes operations which takes private pipe data as an argument.
Thus this code can be cleaned and all references to RPC inode can be replaced
with privtae pipe data references.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>