ext4: rename EXT4_GET_BLOCKS_PRE_IO

ext4: rename EXT4_GET_BLOCKS_PRE_IO

This flag has been generalized to split an unwritten extent when we do
dio or dioread_nolock writeback, or to avoid merge new extents which was
created by extents split. Update some related comments too.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
Message-ID: <20251112084538.1658232-2-yangerkun@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>