bpf: Make migrate_{disable,enable} always inline if in header file

With latest bpf/bpf-next tree and latest pahole master, I got the following
build failure:

  $ 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]: *** [/home/yhs/work/bpf-next/scripts/Makefile.vmlinux:72: vmlinux.unstripped] Error 255
  make[2]: *** Deleting file 'vmlinux.unstripped'
  make[1]: *** [/home/yhs/work/bpf-next/Makefile:1242: vmlinux] Error 2
  make: *** [/home/yhs/work/bpf-next/Makefile:248: __sub-make] Error 2

In pahole patch [1], if two functions having identical names but different
addresses, then this function name is considered ambiguous and later on
this function will not be added to vmlinux/module BTF.

Commit 378b770 ("sched: Make migrate_{en,dis}able() inline") changed
original global funcitons migrate_{enable,disable} to
  - in kernel/sched/core.c, migrate_{enable,disable} are global funcitons.
  - in other places, migrate_{enable,disable} may survive as static functions
    since they are marked as 'inline' in include/linux/sched.h and the
    'inline' attribute does not garantee inlining.

If I build with clang compiler (make LLVM=1 -j) (llvm21 and llvm22), I found
there are four symbols for migrate_{enable,disable} respectively, three
static functions and one global function. With the above pahole patch [1],
migrate_{enable,disable} are not in vmlinux BTF and this will cause
later resolve_btfids failure.

Making migrate_{enable,disable} always inline in include/linux/sched.h
can fix the problem.

  [1] https://lore.kernel.org/dwarves/[email protected]/

Fixes: 378b770 ("sched: Make migrate_{en,dis}able() inline")
Cc: Menglong Dong <[email protected]>
Cc: Ihor Solodrai <[email protected]>
Signed-off-by: Yonghong Song <[email protected]>
Acked-by: Menglong Dong <[email protected]>

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();
 }