SUNRPC: Revert 561141dd

Scott reports an occasional scatterlist BUG that is triggered by the
RFC 8009 Kunit test, then says:

> Looking through the git history of the auth_gss code, there are various
> places where static buffers were replaced by dynamically allocated ones
> because they're being used with scatterlists.

Reported-by: Scott Mayhew <smayhew@redhat.com>
Fixes: 561141dd

 ("SUNRPC: Use a static buffer for the checksum initialization vector")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>