Fix: btrfs instrumentation namespacing

Fix: btrfs instrumentation namespacing

Triggers this warning:

[   47.922818] WARNING: CPU: 29 PID: 1163 at /home/efficios/git/lttng-modules/lttng-probes.c:83 fixup_lazy_probes+0x1fb/0x210 [lttng_tracer]
[   47.951661] 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_uprobes(O) lttng_lib_ring_buffer(O) lttng_kretprobes(O)
[   48.039200] CPU: 29 PID: 1163 Comm: modprobe Tainted: G           O      4.19.0-rc7+ #19
[   48.055628] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[   48.078273] RIP: 0010:fixup_lazy_probes+0x1fb/0x210 [lttng_tracer]
[   48.092257] Code: 01 48 39 d0 76 24 43 80 3c 3a 5f 75 1d 44 8b 44 24 14 4c 8b 4c 24 08 41 83 c0 01 45 39 45 10 0f 86 7a fe ff ff e9 6c ff ff ff <0f> 0b e9 6e fe ff ff 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 0f
[   48.141947] RSP: 0018:ffffafe74777bc40 EFLAGS: 00010286
[   48.153733] RAX: 00000000ffffffff RBX: dead000000000200 RCX: 0000000000000061
[   48.173986] RDX: 0000000000000005 RSI: ffffffffc04b728c RDI: ffffffffc04b74a5
[   48.193595] RBP: dead000000000100 R08: 0000000000000062 R09: ffffffffc04bd040
[   48.211573] R10: ffffffffc04b74a5 R11: ffffffff920e204d R12: ffffffffffffffff
[   48.232131] R13: ffffffffc04bd000 R14: ffffffffc03f0078 R15: 0000000000000005
[   48.246832] FS:  00007f5495093540(0000) GS:ffff8fcf0fb40000(0000) knlGS:0000000000000000
[   48.267475] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   48.280348] CR2: 00007ffde00f8328 CR3: 00000018078a2001 CR4: 00000000001606e0
[   48.302404] Call Trace:
[   48.309201]  lttng_probe_register+0xd5/0xe0 [lttng_tracer]
[   48.326993]  ? __event_probe__compaction_isolate_template+0x2c0/0x2c0 [lttng_probe_compaction]
[   48.345702]  do_one_initcall+0x46/0x1c8
[   48.360147]  ? kobject_uevent_env+0x117/0x810
[   48.370388]  ? _cond_resched+0x15/0x40
[   48.380649]  ? kmem_cache_alloc_trace+0x153/0x1c0
[   48.394706]  do_init_module+0x5b/0x20b
[   48.404412]  load_module+0x2194/0x2980
[   48.418759]  ? ima_post_read_file+0xe2/0x120
[   48.427716]  ? __do_sys_finit_module+0xe9/0x110
[   48.438226]  __do_sys_finit_module+0xe9/0x110
[   48.452983]  do_syscall_64+0x65/0x190
[   48.461521]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   48.473173] RIP: 0033:0x7f5494ba6839
[   48.486630] Code: 00 f3 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 1f f6 2c 00 f7 d8 64 89 01 48
[   48.529108] RSP: 002b:00007ffde00fb408 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[   48.549652] RAX: ffffffffffffffda RBX: 000055d326ee2ac0 RCX: 00007f5494ba6839
[   48.564325] RDX: 0000000000000000 RSI: 000055d326cbac2e RDI: 0000000000000005
[   48.582576] RBP: 000055d326cbac2e R08: 0000000000000000 R09: 000055d326ee2ac0
[   48.596892] R10: 0000000000000005 R11: 0000000000000246 R12: 0000000000000000
[   48.617576] R13: 000055d326ee2d80 R14: 0000000000040000 R15: 000055d326ee2ac0
[   48.633713] ---[ end trace c265591e0ada440c ]---

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