Handle block pointers with a corrupt logical size

Commit 5f6d0b6 was originally added to gracefully handle block
pointers with a damaged logical size.  However, it incorrectly
assumed that all passed arc_done_func_t could handle a NULL
arc_buf_t.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4069
Closes #4080

Author: behlendorf

module/zfs/arc.c

@@ -4319,17 +4319,11 @@ arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp, arc_done_func_t *done,
 
 		/*
 		 * Gracefully handle a damaged logical block size as a
-		 * checksum error by passing a dummy zio to the done callback.
+		 * checksum error.
 		 */
 		if (size > spa_maxblocksize(spa)) {
-			if (done) {
-				rzio = zio_null(pio, spa, NULL,
-				    NULL, NULL, zio_flags);
-				rzio->io_error = ECKSUM;
-				done(rzio, buf, private);
-				zio_nowait(rzio);
-			}
-			rc = ECKSUM;
+			ASSERT3P(buf, ==, NULL);
+			rc = SET_ERROR(ECKSUM);
 			goto out;
 		}