bpf: Fix distinct pointer types warning for ARCH=i386

Fix a new warning reported by kbuild for make ARCH=i386:

   In file included from kernel/bpf/cgroup.c:11:0:
   kernel/bpf/cgroup.c: In function '__cgroup_bpf_run_filter_sysctl':
   include/linux/kernel.h:827:29: warning: comparison of distinct pointer types lacks a cast
      (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
                                ^
   include/linux/kernel.h:841:4: note: in expansion of macro '__typecheck'
      (__typecheck(x, y) && __no_side_effects(x, y))
       ^~~~~~~~~~~
   include/linux/kernel.h:851:24: note: in expansion of macro '__safe_cmp'
     __builtin_choose_expr(__safe_cmp(x, y), \
                           ^~~~~~~~~~
   include/linux/kernel.h:860:19: note: in expansion of macro '__careful_cmp'
    #define min(x, y) __careful_cmp(x, y, <)
                      ^~~~~~~~~~~~~
>> kernel/bpf/cgroup.c:837:17: note: in expansion of macro 'min'
      ctx.new_len = min(PAGE_SIZE, *pcount);
                    ^~~

Fixes: 4e63acdf ("bpf: Introduce bpf_sysctl_{get,set}_new_value helpers")
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>