ext4: implement mmap path using iomap

Introduce ext4_iomap_page_mkwrite() to implement the mmap iomap path
for ext4. The heavy lifting is delegated to iomap_page_mkwrite(), which
only requires ext4_iomap_buffered_write_ops and
ext4_iomap_buffered_da_write_ops to allocate and map blocks.

Note that the lock ordering between folio lock and transaction start in
this path is reversed compared to the buffer_head buffered write path.
The lock ordering documentation in super.c has been updated accordingly.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: syzbot@syzkaller.appspotmail.com
Link: https://patch.msgid.link/20261009103033.2920530-17-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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