bpf: Make migrate_disable always inline to avoid partial inlining

The build fails with llvm 21/22:

  $ make LLVM=1 -j
    ...
    LD      vmlinux.o
    GEN     .vmlinux.objs
    ...
    BTF     .tmp_vmlinux1.btf.o
    ...
    AS      .tmp_vmlinux2.kallsyms.o
    LD      vmlinux.unstripped
    BTFIDS  vmlinux.unstripped
  WARN: resolve_btfids: unresolved symbol migrate_enable
  WARN: resolve_btfids: unresolved symbol migrate_disable
  make[2]: *** [vmlinux.unstripped] Error 255
  make[2]: *** Deleting file 'vmlinux.unstripped'
  make[1]: *** [Makefile:1242: vmlinux] Error 2
  make: *** [Makefile:248: __sub-make] Error 2

Two functions with identical names but different addresses are
considered ambiguous and removed by "pahole" from vmlinux BTF.
Later resolve_btfids warns since it cannot find them.

Commit 378b770 ("sched: Make migrate_{en,dis}able() inline") made
them inlineable in most places, but in vmlinux built with llvm 21 and 22
there are four symbols for migrate_{enable,disable}:
three static functions and one global function.

Fix the issue by marking migrate_{enable,disable} as always inline.
The alternative is to mark them as notrace/nokprobe which is more
drastic. Only bpf programs are prevented from attaching to these
functions. The rest of the tracing shouldn't be affected.

[note: Peter ok-ed the patch, Alexei rewrote commit log]

Fixes: 378b770 ("sched: Make migrate_{en,dis}able() inline")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Menglong Dong <menglong.dong@linux.dev>
Link: https://lore.kernel.org/r/20251029183646.3811774-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Yonghong Song

include/linux/sched.h

@@ -2407,12 +2407,12 @@ static inline void __migrate_enable(void) { }
  * be defined in kernel/sched/core.c.
  */
 #ifndef INSTANTIATE_EXPORTED_MIGRATE_DISABLE
-static inline void migrate_disable(void)
+static __always_inline void migrate_disable(void)
 {
 	__migrate_disable();
 }
 
-static inline void migrate_enable(void)
+static __always_inline void migrate_enable(void)
 {
 	__migrate_enable();
 }