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>