ext4: implement writeback path using iomap

Add the iomap writeback path for ext4 buffered I/O. This introduces:

 - ext4_iomap_writepages(): the main writeback entry point.
 - ext4_writeback_ops: a new iomap_writeback_ops instance to handle
   block mapping and I/O submission.
 - A new end I/O work handler ext4_iomap_finish_ioend() is added for
   converting unwritten extents, updating file size, and handling
   DATA_ERR_ABORT after I/O completion.

Core implementation details:

 - ->writeback_range() callback
   Calls ext4_iomap_map_writeback_range() to map and allocate blocks
   using the enhanced ext4_map_blocks(). ext4_map_blocks() now starts
   its own transaction internally when it needs to allocate blocks.
   For performance, when a block range is not yet allocated, it
   allocates based on the writeback length and delalloc extent length
   (ext4_map_blocks() will trim the allocated length), rather than
   allocating a single folio at a time. The folio is then added to
   an iomap_ioend instance.

 - ->writeback_submit() callback
   Registers ext4_iomap_end_bio() as the end bio callback. This callback
   schedules a worker to handle:
   - Unwritten extent conversion.
   - i_disksize update after data is written back.
   - Journal abort on writeback I/O failure.

 - The new end I/O work handler ext4_iomap_finish_ioend()
   This reuses i_rsv_conversion_work and i_rsv_conversion_list, which
   are now used in the buffer_head writeback path for memory saving,
   because an inode can never go through both buffer_head and iomap
   paths. Those are not initialized in this patch but will be when the
   iomap path is formally enabled later.

Key changes and considerations:

 - Append write and unwritten extents
   Since data=ordered mode is not used to prevent stale data exposure
   during append writebacks, new blocks are always allocated as
   unwritten extents (i.e. always enable dioread_nolock), and i_disksize
   update is postponed until I/O completion. Additionally, the deadlock
   that the reserve handle was expected to resolve does not occur
   anymore. Therefore, the end I/O worker can start a normal journal
   handle instead of a reserve handle when converting unwritten extents.

 - Lock ordering
   The ->writeback_range() callback runs under the folio lock, requiring
   the journal handle to be started under that same lock. This reverses
   the order compared to the buffer_head writeback path. The lock
   ordering documentation in super.c has been updated accordingly.

 - Don't cache writes
   The iomap infrastructure sets the BIO_COMPLETE_IN_TASK flag when
   submitting I/O, so the ioend will be processed in task context.
   However, if a private defer worker is to be started, this flag must
   be cleared explicitly to avoid double deferral. In the future, all
   private defer work should be moved to the generic bio complete in
   task framework.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Tested-by: syzbot@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20261009103033.2920530-16-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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