Fix: btrfs instrumentation namespacing

Fix: btrfs instrumentation namespacing

Trips this warning:

[  122.301894] WARNING: CPU: 6 PID: 1654 at /home/efficios/git/lttng-modules/lttng-probes.c:99 fixup_lazy_probes+0x195/0x200 [lttng_tracer]
[  122.304974] Modules linked in: lttng_probe_compaction(O+) lttng_probe_btrfs(O) lttng_probe_block(O) lttng_ring_buffer_metadata_mmap_client(O) lttng_ring_buffer_client_mmap_overwrite(O) lttng_ring_buffer_client_mmap_discard(O) lttng_ring_buffer_metadata_client(O) lttng_ring_buffer_client_overwrite(O) lttng_ring_buffer_client_discard(O) lttng_tracer(O) lttng_statedump(O) lttng_ftrace(O) lttng_kprobes(O) lttng_clock(O) lttng_lib_ring_buffer(O) lttng_kretprobes(O)
[  122.314772] CPU: 6 PID: 1654 Comm: modprobe Tainted: G           O     4.16.0-rc6+ #54
[  122.316738] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[  122.320280] RIP: 0010:fixup_lazy_probes+0x195/0x200 [lttng_tracer]
[  122.321825] RSP: 0018:ffffc90008467ca0 EFLAGS: 00010286
[  122.323137] RAX: 00000000ffffffff RBX: ffffffffa01e7000 RCX: 0000000000000061
[  122.324847] RDX: 0000000000000005 RSI: ffffffffa01e21ac RDI: ffffffffa01e233b
[  122.326528] RBP: ffffffffa017f078 R08: 0000000000000062 R09: 0000000000000345
[  122.328154] R10: 0000000000000000 R11: ffffc90008467a28 R12: 0000000000000005
[  122.329791] R13: 0000000000000010 R14: 0000000000000010 R15: 0000000000000006
[  122.331410] FS:  00007f6c8d9a7740(0000) GS:ffff880c0fb80000(0000) knlGS:0000000000000000
[  122.333323] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  122.334673] CR2: 00007ffcc9698ff8 CR3: 0000000c0afae004 CR4: 00000000001606e0
[  122.336300] Call Trace:
[  122.337011]  ? __event_probe__compaction_migratepages+0x250/0x250 [lttng_probe_compaction]
[  122.338901]  lttng_get_probe_list_head.part.2+0x19/0x20 [lttng_tracer]
[  122.340349]  lttng_probe_register+0xd5/0xe0 [lttng_tracer]
[  122.341607]  ? __event_probe__compaction_migratepages+0x250/0x250 [lttng_probe_compaction]
[  122.343453]  do_one_initcall+0x3d/0x16e
[  122.344383]  ? _cond_resched+0x15/0x30
[  122.345323]  ? kmem_cache_alloc_trace+0xe1/0x1b0
[  122.346394]  ? do_init_module+0x22/0x20c
[  122.347329]  do_init_module+0x5a/0x20c
[  122.350037]  load_module+0x244f/0x2980
[  122.350958]  ? m_show+0x190/0x190
[  122.351774]  ? security_capable+0x41/0x60
[  122.352723]  SYSC_finit_module+0x80/0xb0
[  122.353716]  do_syscall_64+0x76/0x1a0
[  122.354565]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[  122.355669] RIP: 0033:0x7f6c8d4c73c9
[  122.356502] RSP: 002b:00007ffcc969c248 EFLAGS: 00000206 ORIG_RAX: 0000000000000139
[  122.358209] RAX: ffffffffffffffda RBX: 000055763df4fee9 RCX: 00007f6c8d4c73c9
[  122.359684] RDX: 0000000000000000 RSI: 000055763df4fee9 RDI: 0000000000000004
[  122.361182] RBP: 0000000000000000 R08: 0000000000000000 R09: 000055763f39a450
[  122.362663] R10: 0000000000000004 R11: 0000000000000206 R12: 000055763f392400
[  122.364144] R13: 000055763f396cb0 R14: 000055763f3925a0 R15: 0000000000040000
[  122.365690] Code: 25 14 a0 4a 8b 04 f0 48 8b 30 31 c0 e8 25 3b 10 e1 48 8b 43 08 48 8b 33 4c 89 e2 4a 8b 04 f0 48 8b 38 e8 9f b7 b1 e1 85 c0 74 07 <0f> 0b e9 b3 fe ff ff 48 c7 c7 16 26 14 a0 e8 f8 3a 10 e1 48 8b
[  122.369348] ---[ end trace 15840f1166edf835 ]---

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>