zfs: com.delphix:hole_birth feature support

In the past birth was always zero for holes. This feature started
to make use of birth for holes as well, so change code to test for
valid DVA address instead.

grub-core/fs/zfs/zfs.c

@@ -280,7 +280,9 @@ grub_crypto_cipher_handle_t (*grub_zfs_load_key) (const struct grub_zfs_key *key
  */
 #define MAX_SUPPORTED_FEATURE_STRLEN 50
 static const char *spa_feature_names[] = {
-  "org.illumos:lz4_compress",NULL
+  "org.illumos:lz4_compress",
+  "com.delphix:hole_birth",
+  NULL
 };
 
 static int
@@ -1751,7 +1753,7 @@ zio_read_gang (blkptr_t * bp, grub_zfs_endian_t endian, dva_t * dva, void *buf,
 
   for (i = 0; i < SPA_GBH_NBLKPTRS; i++)
     {
-      if (zio_gb->zg_blkptr[i].blk_birth == 0)
+      if (BP_IS_HOLE(&zio_gb->zg_blkptr[i]))
 	continue;
 
       err = zio_read_data (&zio_gb->zg_blkptr[i], endian, buf, data);