ext4: get rid of ppath in get_ext_path()

ext4: get rid of ppath in get_ext_path()

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.

After getting rid of ppath in get_ext_path(), its caller may pass an error
pointer to ext4_free_ext_path(), so it needs to teach ext4_free_ext_path()
and ext4_ext_drop_refs() to skip the error pointer. 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/20240822023545.1994557-13-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>