ext4: implement partial block zero range path using iomap

Introduce a new iomap_ops instance, ext4_iomap_zero_ops, along with
ext4_block_iomap_zero_range() to implement block zeroing via the iomap
infrastructure for ext4.

ext4_block_iomap_zero_range() calls iomap_zero_range() with
ext4_iomap_zero_begin() as the callback. The callback locates the
range and populates the iomap mapping. If the range is mapped,
iomap_zero_iter() in the iomap core zeros the partial block
directly. If the range is an unwritten extent within EOF, the
callback collects a dirty folio batch via iomap_fill_dirty_folios()
so that iomap_zero_iter() can zero those folios directly, bypassing
a separate slow flush operation that would otherwise be needed to
convert the unwritten extent.

Note that ext4_iomap_zero_begin() can race with concurrent writeback:
after it queries an unwritten extent, writeback may convert it to
written and complete on the folio before iomap_fill_dirty_folios() scans
the range. The empty batch then makes iomap_zero_iter() skip zeroing,
leaving stale on-disk data.

  zero_range                      writeback
  ----------------------          ----------------------
  ext4_block_zero_range()
    iomap_zero_range()
      iomap_iter()
        ext4_iomap_zero_begin()
          ext4_iomap_map_blocks()
            -> extent is UNWRITTEN
                                  ext4_convert_unwritten_extents_endio()
                                    -> extent is converted to WRITTEN
                                    -> folio is clean
          iomap_fill_dirty_folios()
            filemap_get_folios_dirty()
            -> folio is clean, not added to batch
          ext4_set_iomap() -> IOMAP_UNWRITTEN
      iomap_zero_iter()
        __iomap_get_folio() -> NULL (empty batch)
        iomap_iter_advance_full()  <-- zeroing skipped

  [later read returns stale on-disk data]   <--   CORRUPTION

Therefore, we retry the extent lookup when iomap_fill_dirty_folios()
adds nothing and the i_es_seq cookie captured at the first lookup has
advanced, indicating the race actually occurred.

Other important constraints:

Zeroing out under an active journal handle can cause deadlock, as the
lock/handle ordering is inconsistent with the iomap writeback path.
Therefore, ext4_block_iomap_zero_range() must not be called under an
active handle. In addition, for post-EOF zeroing, the caller cannot
rely on data=ordered mode to persist the zeroed data before
i_disksize is updated.

Subsequent patches will address this by deferring i_disksize update
to i_size until after the zeroed data has been written back.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Tested-by: syzbot@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20261009103033.2920530-18-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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