udev-extraconf: fix unmount directories containing octal-escaped chars

udev-extraconf: fix unmount directories containing octal-escaped chars

USB devices are auto-mounted in a directory named like theirs labels.
Special characters like whitespace are octal-escaped in /proc/mounts
output. Using directly this output file as an argument for umount failed
and the mount directory can't be removed as still busy.
Using printf allows these special characters to be unescaped.

Signed-off-by: Jonathan GUILLOT <jonathan@joggee.fr>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>