Map lookup and deletion can yield stale keys and values by virtue of
sharing a data structure with userspace programs and other BPF programs
which can modify it. However, all accesses remain perfectly safe and will
not cause memory corruption or data races.
This is necessary since the context is used in many other program types
and not just in SK_SKB programs.
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>