GCC 7 appears to be clever enough to detect that transient_len is
uninitialised but not that it won't be used despite [1]. Just initialise
it to zero to stop the complaining, and allow LXC to build on openSUSE
Leap.

[1]: 34683042 ("cgroups: fix "uninitialized transient_len" warning")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>