bpf: Fix pointer-to-int-cast warning in newer Clang

[pchaigno]

Latest Clang (9658d895 "[Sema] Adds the pointer-to-int-cast diagnostic") emits the following warning for our __fetch code:

In file included from bpf_lxc.c:18:
In file included from /home/paul/cilium/bpf/lib/maps.h:8:
In file included from /home/paul/cilium/bpf/lib/ipv6.h:9:
/home/paul/cilium/bpf/lib/dbg.h:155:13: error: cast to smaller integer type '__u32' (aka 'unsigned int') from '__u32 *' (aka 'unsigned int *') [-Werror,-Wpointer-to-int-cast]
                .source = EVENT_SOURCE,
                          ^~~~~~~~~~~~
bpf_lxc.c:12:22: note: expanded from macro 'EVENT_SOURCE'
#define EVENT_SOURCE LXC_ID
                     ^~~~~~
/home/paul/cilium/bpf/lxc_config.h:14:16: note: expanded from macro 'LXC_ID'
#define LXC_ID fetch_u32(LXC_ID)
               ^~~~~~~~~~~~~~~~~
/home/paul/cilium/bpf/lib/static_data.h:11:22: note: expanded from macro 'fetch_u32'
#define fetch_u32(x) __fetch(x)
                     ^~~~~~~~~~
/home/paul/cilium/bpf/include/bpf/ctx/../compiler.h:46:22: note: expanded from macro '__fetch'
# define __fetch(X)             (__u32)(&(X))
                                ^~~~~~~~~~~~~

Fix it by casting to __u64 first.

---

This change is 

[coveralls]

Coverage decreased (-0.01%) to 45.624% when pulling 07ce43d on pr/pchaigno/fix-ptr-to-int-warning into 8ea7239 on master.

[brb]

test-me-please