SUNRPC: Fix failures of checksum Kunit tests

Scott reports that when the new GSS krb5 Kunit tests are built as
a separate module and loaded, the RFC 6803 and RFC 8009 checksum
tests all fail, even though they pass when run under kunit.py.

It appears that passing a buffer backed by static const memory to
gss_krb5_checksum() is a problem. A printk in checksum_case() shows
the correct plaintext, but by the time the buffer has been converted
to a scatterlist and arrives at checksummer(), it contains all
zeroes.

Replacing this buffer with one that is dynamically allocated fixes
the issue.
Reported-by: Scott Mayhew <smayhew@redhat.com>
Fixes: 02142b2c ("SUNRPC: Add checksum KUnit tests for the RFC 6803 encryption types")
Tested-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/auth_gss/gss_krb5_test.c

@@ -73,7 +73,6 @@ static void checksum_case(struct kunit *test)
 {
 	const struct gss_krb5_test_param *param = test->param_value;
 	struct xdr_buf buf = {
-		.head[0].iov_base	= param->plaintext->data,
 		.head[0].iov_len	= param->plaintext->len,
 		.len			= param->plaintext->len,
 	};
@@ -99,6 +98,10 @@ static void checksum_case(struct kunit *test)
 	err = crypto_ahash_setkey(tfm, Kc.data, Kc.len);
 	KUNIT_ASSERT_EQ(test, err, 0);
 
+	buf.head[0].iov_base = kunit_kzalloc(test, buf.head[0].iov_len, GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buf.head[0].iov_base);
+	memcpy(buf.head[0].iov_base, param->plaintext->data, buf.head[0].iov_len);
+
 	checksum.len = gk5e->cksumlength;
 	checksum.data = kunit_kzalloc(test, checksum.len, GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, checksum.data);