ext4: remove useless ext4_iomap_overwrite_ops

ext4: remove useless ext4_iomap_overwrite_ops

ext4_iomap_overwrite_ops was introduced in commit 8cd115bdda17 ("ext4:
Optimize ext4 DIO overwrites"), which can optimize pure overwrite
performance by dropping the IOMAP_WRITE flag to only query the mapped
mapping information. This avoids starting a new journal handle, thereby
improving speed. Later, commit 9faac62d4013 ("ext4: optimize file
overwrites") also optimized similar scenarios, but it performs the check
later, examining the mappings status only when the actual block mapping
is needed. Thus, it can handle the previous commit scenario. That means
in the case of an overwrite scenario, the condition
"offset + length <= i_size_read(inode)" in the write path must always be
true.

Therefore, it is acceptable to remove the ext4_iomap_overwrite_ops,
which will also clarify the write and read paths of ext4_iomap_begin.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/20260105014522.1937690-5-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>