bpf: Finish constification of 1st parameter of bpf_d_path()

The commit 1b8abbb1 ("bpf...d_path(): constify path argument")
constified the first parameter of the bpf_d_path(), but failed to
update it in all places. Finish constification.

Otherwise the selftest fail to build:
.../selftests/bpf/bpf_experimental.h:222:12: error: conflicting types for 'bpf_path_d_path'
  222 | extern int bpf_path_d_path(const struct path *path, char *buf, size_t buf__sz) __ksym;
      |            ^
.../selftests/bpf/tools/include/vmlinux.h:153922:12: note: previous declaration is here
 153922 | extern int bpf_path_d_path(struct path *path, char *buf, size_t buf__sz) __weak __ksym;

Fixes: 1b8abbb1 ("bpf...d_path(): constify path argument")
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>