Wait for txg sync if the last DRR_FREEOBJECTS might result in a hole

If we receive a DRR_FREEOBJECTS as the first entry in an object range,
this might end up producing a hole if the freed objects were the
only existing objects in the block.

If the txg starts syncing before we've processed any following
DRR_OBJECT records, this leads to a possible race where the backing
arc_buf_t gets its psize set to 0 in the arc_write_ready() callback
while still being referenced from a dirty record in the open txg.

To prevent this, we insert a txg_wait_synced call if the first
record in the range was a DRR_FREEOBJECTS that actually
resulted in one or more freed objects.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: David Hedberg <david.hedberg@findity.com>
Sponsored by: Findity AB
Closes #11893
Closes #14358

tests/runfiles/common.run

@@ -824,9 +824,9 @@ tests = ['recv_dedup', 'recv_dedup_encrypted_zvol', 'rsend_001_pos',
     'send-c_mixed_compression', 'send-c_stream_size_estimate',
     'send-c_embedded_blocks', 'send-c_resume', 'send-cpL_varied_recsize',
     'send-c_recv_dedup', 'send-L_toggle',
-    'send_encrypted_incremental.ksh', 'send_encrypted_hierarchy',
-    'send_encrypted_props', 'send_encrypted_truncated_files',
-    'send_freeobjects', 'send_realloc_files',
+    'send_encrypted_incremental.ksh', 'send_encrypted_freeobjects',
+    'send_encrypted_hierarchy', 'send_encrypted_props',
+    'send_encrypted_truncated_files', 'send_freeobjects', 'send_realloc_files',
     'send_realloc_encrypted_files', 'send_spill_block', 'send_holds',
     'send_hole_birth', 'send_mixed_raw', 'send-wR_encrypted_zvol',
     'send_partial_dataset', 'send_invalid', 'send_doall',

tests/runfiles/sanity.run

@@ -547,6 +547,7 @@ tests = ['recv_dedup', 'recv_dedup_encrypted_zvol', 'rsend_001_pos',
     'rsend_014_pos', 'rsend_016_neg', 'send-c_verify_contents',
     'send-c_volume', 'send-c_zstreamdump', 'send-c_recv_dedup',
     'send-L_toggle', 'send_encrypted_hierarchy', 'send_encrypted_props',
+    'send_encrypted_freeobjects',
     'send_encrypted_truncated_files', 'send_freeobjects', 'send_holds',
     'send_mixed_raw', 'send-wR_encrypted_zvol', 'send_partial_dataset',
     'send_invalid']