x86/mm: Rename is_kernel_text to __is_kernel_text

x86/mm: Rename is_kernel_text to __is_kernel_text

The kbuild test robot reported compile issue on x86 in one of
the following patches that adds <linux/kallsyms.h> include into
<linux/bpf.h>, which is picked up by init_32.c object.

The problem is that <linux/kallsyms.h> defines global function
is_kernel_text which colides with the static function of the
same name defined in init_32.c:

  $ make ARCH=i386
  ...
  >> arch/x86/mm/init_32.c:241:19: error: redefinition of 'is_kernel_text'
    static inline int is_kernel_text(unsigned long addr)
                      ^~~~~~~~~~~~~~
   In file included from include/linux/bpf.h:21:0,
                    from include/linux/bpf-cgroup.h:5,
                    from include/linux/cgroup-defs.h:22,
                    from include/linux/cgroup.h:28,
                    from include/linux/hugetlb.h:9,
                    from arch/x86/mm/init_32.c:18:
   include/linux/kallsyms.h:31:19: note: previous definition of 'is_kernel_text' was here
    static inline int is_kernel_text(unsigned long addr)

Renaming the init_32.c is_kernel_text function to __is_kernel_text.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200312195610.346362-2-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>