ext4: add iomap address space operations for buffered I/O

Introduce initial support for iomap in the buffered I/O path for regular
files on ext4.

  - Add a new inode state flag EXT4_STATE_BUFFERED_IOMAP to indicate the
    inode uses iomap instead of buffer_head for buffered I/O
  - Add helper ext4_inode_buffered_iomap() to check the flag
  - Add new address space operations ext4_iomap_aops with callbacks that
    will use generic iomap implementations
  - Add ext4_iomap_aops to ext4_set_aops() when the flag is set

The following callbacks(read_folio(), readahead(), writepages()) are
provided as placeholders and will be implemented in later patches.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Tested-by: syzbot@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20261009103033.2920530-10-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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