SUNRPC: Use a static buffer for the checksum initialization vector

Allocating and zeroing a buffer during every call to
krb5_etm_checksum() is inefficient. Instead, set aside a static
buffer that is the maximum crypto block size, and use a portion
(or all) of that.

Reported-by: Markus Elfring <Markus.Elfring@web.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>