ext4: remove unused member 'i_unwritten' from 'ext4_inode_info'

ext4: remove unused member 'i_unwritten' from 'ext4_inode_info'

After commit 378f32bab371 ("ext4: introduce direct I/O write using iomap
infrastructure"), no one cares about the value of i_unwritten, so there
is no need to maintain this variable, remove it, and clean up the
associated logic.

Suggested-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250122110533.4116662-9-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>