modprobe.d: fix directory of modprobe configuration file (#7149)

kmod upstream uses /lib/modprobe.d which means we need to use rootprefix
instead of prefix for installing the modprobe configuration file as
otherwise split-usr systems are broken.

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=879191

(cherry picked from commit 02fa054d)