ext4: dax: fix overflowing extents beyond inode size when partially writing

The dax_iomap_rw() does two things in each iteration: map written blocks
and copy user data to blocks. If the process is killed by user(See signal
handling in dax_iomap_iter()), the copied data will be returned and added
on inode size, which means that the length of written extents may exceed
the inode size, then fsck will fail. An example is given as:

dd if=/dev/urandom of=file bs=4M count=1
 dax_iomap_rw
  iomap_iter // round 1
   ext4_iomap_begin
    ext4_iomap_alloc // allocate 0~2M extents(written flag)
  dax_iomap_iter // copy 2M data
  iomap_iter // round 2
   iomap_iter_advance
    iter->pos += iter->processed // iter->pos = 2M
   ext4_iomap_begin
    ext4_iomap_alloc // allocate 2~4M extents(written flag)
  dax_iomap_iter
   fatal_signal_pending
  done = iter->pos - iocb->ki_pos // done = 2M
 ext4_handle_inode_extension
  ext4_update_inode_size // inode size = 2M

fsck reports: Inode 13, i_size is 2097152, should be 4194304.  Fix?

Fix the problem by truncating extents if the written length is smaller
than expected.

Fixes: 776722e8 ("ext4: DAX iomap write support")
CC: stable@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=219136


Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Link: https://patch.msgid.link/20240809121532.2105494-1-chengzhihao@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>