sunrpc: Fix RFC6803 encryption test

The usage_data[] array in rfc6803_encrypt_case() is uninitialised, so clear
it as it may cause the tests to fail otherwise.

Fixes: b958cff6 ("SUNRPC: Add encryption KUnit tests for the RFC 6803 encryption types")
Link: https://lore.kernel.org/r/380323.1681314997@warthog.procyon.org.uk/


Signed-off-by: David Howells <dhowells@redhat.com>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Scott Mayhew <smayhew@redhat.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: linux-nfs@vger.kernel.org
cc: linux-crypto@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>