Raw DRR_OBJECT records must write raw data

b1d21733 made it possible for empty metadnode blocks to be
compressed to a hole, fixing a bug that would cause invalid
metadnode MACs when a send stream attempted to free objects
and allowing the blocks to be reclaimed when they were no
longer needed. However, this patch also introduced a race
condition; if a txg sync occurred after a DRR_OBJECT_RANGE
record was received but before any objects were added, the
metadnode block would be compressed to a hole and lose all
of its encryption parameters. This would cause subsequent
DRR_OBJECT records to fail when they attempted to write
their data into an unencrypted block. This patch defers the
DRR_OBJECT_RANGE handling to receive_object() so that the
encryption parameters are set with each object that is
written into that block.

Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7215 
Closes #7236

include/sys/dmu.h

@@ -821,8 +821,9 @@ void dmu_assign_arcbuf_by_dnode(dnode_t *dn, uint64_t offset,
 void dmu_assign_arcbuf_by_dbuf(dmu_buf_t *handle, uint64_t offset,
     struct arc_buf *buf, dmu_tx_t *tx);
 #define	dmu_assign_arcbuf	dmu_assign_arcbuf_by_dbuf
-void dmu_convert_to_raw(dmu_buf_t *handle, boolean_t byteorder,
-    const uint8_t *salt, const uint8_t *iv, const uint8_t *mac, dmu_tx_t *tx);
+int dmu_convert_mdn_block_to_raw(objset_t *os, uint64_t firstobj,
+    boolean_t byteorder, const uint8_t *salt, const uint8_t *iv,
+    const uint8_t *mac, dmu_tx_t *tx);
 void dmu_copy_from_buf(objset_t *os, uint64_t object, uint64_t offset,
     dmu_buf_t *handle, dmu_tx_t *tx);
 #ifdef HAVE_UIO_ZEROCOPY