systemd: re-enable mount propagation for udevd

With MountFlags=slave, those mounts then become private to the systemd-udevd
namespace and are no longer accessible from outside the namespace, which is
not expected

[YOCTO #8613]

(From OE-Core rev: 73f43d85)

Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f2092e67ea880301058396b831a9a18905317d0d)
Signed-off-by: Alejandro Hernandez <alejandro.hernandez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>