ext4: add missing down_write_data_sem in mext_move_extent().

Commit 962e8a0 ("ext4: introduce mext_move_extent()") attempts to
call ext4_swap_extents() on the failure path to recover the swapped
extents, but fails to acquire locks for the two inode->i_data_sem,
triggering the BUG_ON statement in ext4_swap_extents().

This issue can be fixed by calling ext4_double_down_write_data_sem()
before ext4_swap_extents().

Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
Reported-by: syzbot+4ea6bd8737669b423aae@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69368649.a70a0220.38f243.0093.GAE@google.com/
Fixes: 962e8a0 ("ext4: introduce mext_move_extent()")
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Link: https://patch.msgid.link/20251208123713.1971068-1-sunjunchao@bytedance.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: Julian Sun

fs/ext4/move_extent.c

@@ -393,9 +393,11 @@ static int mext_move_extent(struct mext_data *mext, u64 *m_len)
 
 repair_branches:
 	ret2 = 0;
+	ext4_double_down_write_data_sem(orig_inode, donor_inode);
 	r_len = ext4_swap_extents(handle, donor_inode, orig_inode,
 				  mext->donor_lblk, orig_map->m_lblk,
 				  *m_len, 0, &ret2);
+	ext4_double_up_write_data_sem(orig_inode, donor_inode);
 	if (ret2 || r_len != *m_len) {
 		ext4_error_inode_block(orig_inode, (sector_t)(orig_map->m_lblk),
 				       EIO, "Unable to copy data block, data will be lost!");