DRR_BEGIN payload should use NV_ENCODE_XDR

[tcaputi]

Describe the problem you're observing

While working on verifying the on-disk changes in #6864, I spent some time testing sends from big endian to little endian systems and discovered that (in several cases) they were broken. The source of the problem seems to be that dmu_send.c does not pack the DRR_BEGIN payload in XDR format. As a result, the receive side cannot interpret this payload and returns EOPNOTSUP. This is easy enough to fix with the following patch:

diff --git a/module/zfs/dmu_send.c b/module/zfs/dmu_send.c
index 76b9781..b16d9dd 100644
--- a/module/zfs/dmu_send.c
+++ b/module/zfs/dmu_send.c
@@ -1125,7 +1125,8 @@ dmu_send_impl(void *tag, dsl_pool_t *dp, dsl_dataset_t *to_ds,
                        fnvlist_add_nvlist(nvl, "crypt_keydata", keynvl);
                }
 
-               payload = fnvlist_pack(nvl, &payload_len);
+               VERIFY0(nvlist_pack(nvl, (char **)&payload, &payload_len,
+                   NV_ENCODE_XDR, KM_SLEEP));
                drr->drr_payloadlen = payload_len;
                fnvlist_free(keynvl);
                fnvlist_free(nvl);

The only issue is that this technically constitutes an ABI / format change. The good news, however, is that even older code should be able to understand this change since the nvlist_unpack() function reads the encoding.

Describe how to reproduce the problem

Attempt a resumed or raw send from a big endian system to a little endian one (or visa versa)

[stale]

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