chimera: create missing index i_dirs_iparent

speedups remove operation

Acked-by: Albert Rossi
Acked-by: Paul Millar
Target: master, 2.6, 2.7, 2.8, 2.9, 2.10
Require-book: no
Require-notes: yes
(cherry picked from commit 1db22cf0d017933b64da167c52fe5248ca901733)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Merge pull request #774 from kofemann/fix-missing-index-2.6

chimera: create missing index i_dirs_iparent