SUNRPC: Refactor the GSS-API Per Message calls in the Kerberos mechanism

Replace a number of switches on encryption type so that all of them don't
have to be modified when adding or removing support for an enctype.

Tested-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>