ext4: Make ext4_ext_find_extent fills ext_path completely

When pos=0 or depth, the fields of ext4_ext_path is are not
completely filled.  This patch also removes some unnecessary code.

Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>