meminfo_read: return 0 for Slab

meminfo_read: return 0 for Slab

Slab: is supposed to be the "in-kernel data structures cache".  I don't
know of a good way to calculate this from memory cgroup info.  If/when
we find it we can update it.  This value is used by free -m meaning
that if we don't shrink it, we can end up with negative values for
used memory.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>