ZTS: send-c_volume is flaky

We use block_device_wait to wait for the zvol block device to 
actually appear, and we log the result of the dd calls by using 
an intermediate file.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Wren Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes openzfs#14767

tests/zfs-tests/tests/functional/rsend/send-c_volume.ksh

@@ -29,6 +29,7 @@
 
 function cleanup
 {
+	rm $BACKDIR/copy
 	log_must_busy zfs destroy -r $vol
 	cleanup_pool $POOL2
 }
@@ -60,7 +61,9 @@ log_must eval "zfs recv -d $POOL2 <$BACKDIR/full"
 
 verify_stream_size $BACKDIR/full $vol
 verify_stream_size $BACKDIR/full $vol2
-md5=$(dd if=$voldev2 bs=1024k count=$megs 2>/dev/null | md5digest)
+block_device_wait $voldev2
+log_must dd if=$voldev2 of=$BACKDIR/copy bs=1024k count=$megs
+md5=$(md5digest $BACKDIR/copy)
 [[ $md5 = $md5_1 ]] || log_fail "md5 mismatch: $md5 != $md5_1"
 
 # Repeat, for an incremental send
@@ -72,7 +75,9 @@ log_must eval "zfs recv -d $POOL2 <$BACKDIR/inc"
 
 verify_stream_size $BACKDIR/inc $vol 90 $vol@snap
 verify_stream_size $BACKDIR/inc $vol2 90 $vol2@snap
-md5=$(dd skip=$megs if=$voldev2 bs=1024k count=$megs 2>/dev/null | md5digest)
+block_device_wait $voldev2
+log_must dd skip=$megs if=$voldev2 of=$BACKDIR/copy bs=1024k count=$megs
+md5=$(md5digest $BACKDIR/copy)
 [[ $md5 = $md5_2 ]] || log_fail "md5 mismatch: $md5 != $md5_2"
 
 log_pass "Verify compressed send works with volumes"