jbd2: remove unneeded done_copy_out variable in jbd2_journal_write_metadata_buffer

It's more intuitive to use jh_in->b_frozen_data directly instead of
done_copy_out variable. Simply remove unneeded done_copy_out variable
and use b_frozen_data instead.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240801013815.2393869-6-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>