ext4: implement buffered read path using iomap

Implement the iomap read path for ext4 by introducing a new
ext4_iomap_buffered_read_ops instance. This provides the read_folio()
and readahead() callbacks for ext4_iomap_aops. The implementation
introduces:

 - ext4_iomap_map_blocks(): Helper function to query extent mappings for
   a given read range using ext4_map_blocks() and convert the mapping
   information to iomap type
 - ext4_iomap_buffered_read_begin(): The iomap_begin callbacks that maps
   blocks, validates filesystem state, and populates the iomap. It
   returns -ERANGE for inline data which is not yet supported.

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/20261009103033.2920530-11-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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