btrfs: cleanup duplicated parameters related to create_io_em()

Most parameters of create_io_em() can be replaced by the members with
the same name inside btrfs_file_extent.

Do a direct parameters cleanup here.

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>