btrfs: new define for the inline extent data start

Use a common definition for the inline data start so we don't have to
open-code it and introduce bugs like "Btrfs: fix wrong max inline data
size limit" fixed.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 39021bf..533657c5 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -4625,8 +4625,7 @@
 				ptr = btrfs_item_ptr_offset(leaf, slot);
 				memmove_extent_buffer(leaf, ptr,
 				      (unsigned long)fi,
-				      offsetof(struct btrfs_file_extent_item,
-						 disk_bytenr));
+				      BTRFS_FILE_EXTENT_INLINE_DATA_START);
 			}
 		}