btrfs: do not reset extent map members for inline extents read

Currently for inline extents read inside btrfs_get_extent(), we will
reset several extent map members:

- em->start

  Reset to extent_start, which is completely unnecessary.
  The extent_start and em->start should have already be zero, ensured by
  tree-checker already.

- em->len

  Reset the round_up(copy_size, fs_info->sectorsize), which is again
  unnecessary.

- em->orig_block_len

  Reset to em->len (sectorsize), while it is originally unset from
  btrfs_extent_item_to_extent_map().

  This makes no difference, as all extent map handling paths will
  ignore the orig_block_len if they found it's an inlined extent.

  Such inline extent orig_block_len ignoring examples can be found in
  btrfs_drop_extent_cache().

- em->orig_start

  Reset to em->start (0), while it is originally set to EXTENT_MAP_HOLE.

  This makes no difference either, as all extent map handling paths will
  ignore the em->orig_start if they found it's an inline extent.

Thus all these em members resetting are unnecessary.

Replace them with ASSERT()s checking the only two members (block_start
and length) that make sense.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7082,10 +7082,15 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
 
 		copy_size = min_t(u64, PAGE_SIZE,
 				  btrfs_file_extent_ram_bytes(leaf, item));
-		em->start = extent_start;
-		em->len = ALIGN(copy_size, fs_info->sectorsize);
-		em->orig_block_len = em->len;
-		em->orig_start = em->start;
+
+		/*
+		 * btrfs_extent_item_to_extent_map() should have properly
+		 * initialized em members already.
+		 *
+		 * Other members are not utilized for inline extents.
+		 */
+		ASSERT(em->block_start == EXTENT_MAP_INLINE);
+		ASSERT(em->len = fs_info->sectorsize);
 
 		if (!PageUptodate(page)) {
 			if (btrfs_file_extent_compression(leaf, item) !=