bpf: Fix crash on mm_init trampoline attachment

There are 2 mm_init functions in kernel.

One in kernel/fork.c:
  static struct mm_struct *mm_init(struct mm_struct *mm,
                                   struct task_struct *p,
                                   struct user_namespace *user_ns)

And another one in init/main.c:
  static void __init mm_init(void)

The BTF data will get the first one, which is most likely
(in my case) mm_init from init/main.c without arguments.

Then in runtime when we want to attach to 'mm_init' the
kalsyms contains address of the one from kernel/fork.c.

So we have function model with no arguments and using it
to attach function with 3 arguments.. as result the trampoline
will not save function's arguments and we get crash because
trampoline changes argument registers:

  BUG: unable to handle page fault for address: 0000607d87a1d558
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP PTI
  CPU: 6 PID: 936 Comm: systemd Not tainted 5.12.0-rc4qemu+ torvalds#191
  RIP: 0010:mm_init+0x223/0x2a0
  ...
  Call Trace:
   ? bpf_trampoline_6442453476_0+0x3b/0x1000
   dup_mm+0x66/0x5f0
   ? __lock_task_sighand+0x3a/0x70
   copy_process+0x17d0/0x1b50
   kernel_clone+0x97/0x3c0
   __do_sys_clone+0x60/0x80
   do_syscall_64+0x33/0x40
   entry_SYSCALL_64_after_hwframe+0x44/0xae
  RIP: 0033:0x7f1dc9b3201f

I think there might be more cases like this, but I don't have
an idea yet how to solve this in generic way. The rename in
this change fix it for this instance.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>

init/main.c

@@ -818,7 +818,7 @@ static void __init report_meminit(void)
 /*
  * Set up kernel memory allocators
  */
-static void __init mm_init(void)
+static void __init init_mem(void)
 {
 	/*
 	 * page_ext requires contiguous pages,
@@ -905,7 +905,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
 	vfs_caches_init_early();
 	sort_main_extable();
 	trap_init();
-	mm_init();
+	init_mem();
 
 	ftrace_init();