GEOM: Fix regression after 7f16b501e25.

find_geom() in some classes trim leading "/dev/" from geom names.
Lack of that in geom_gettree_geom() broke some existing scripts.

PR: 262554
MFC after: 2 months

(cherry picked from commit bd0f3d34fa20686bac9a418d3a37a3f59c25ab19)