ext4: get rid of ppath in ext4_split_convert_extents()

ext4: get rid of ppath in ext4_split_convert_extents()

The use of path and ppath is now very confusing, so to make the code more
readable, pass path between functions uniformly, and get rid of ppath.

To get rid of the ppath in ext4_split_convert_extents(), the following is
done here:

 * Its caller needs to update ppath if it uses ppath.

No functional changes.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/20240822023545.1994557-19-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>