btrfs: cleanup duplicated parameters related to btrfs_create_dio_extent()

The following 3 parameters can be cleaned up using btrfs_file_extent
structure:

- len
  btrfs_file_extent::num_bytes

- orig_block_len
  btrfs_file_extent::disk_num_bytes

- ram_bytes
  btrfs_file_extent::ram_bytes

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>