gpart show accepts geom names with the '/dev/' prefix, but gpart list does
not.
This patch modifies find_geom() in sbin/geom/core/geom.c to be consistent with
the behavior of find_geom() in lib/geom/part/geom_part.c
PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=188213