SUNRPC: Use __func__ in dprintk() in auth_gss.c

Clean up: Some function names have changed, but debugging messages
were never updated.  Automate the construction of the function name
in debugging messages.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>