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

The commit 1b8abbb121 ("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: 1b8abbb121 ("bpf...d_path(): constify path argument")
Signed-off-by: Rong Tao <rongtao@cestc.cn>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -4891,7 +4891,7 @@ union bpf_attr {
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
  *
- * long bpf_d_path(struct path *path, char *buf, u32 sz)
+ * long bpf_d_path(const struct path *path, char *buf, u32 sz)
  *	Description
  *		Return full path for given **struct path** object, which
  *		needs to be the kernel BTF *path* object. The path is

scripts/bpf_doc.py

@@ -788,6 +788,7 @@ class PrinterHelpersHeader(Printer):
             'struct task_struct',
             'struct cgroup',
             'struct path',
+            'const struct path',
             'struct btf_ptr',
             'struct inode',
             'struct socket',

tools/include/uapi/linux/bpf.h

@@ -4891,7 +4891,7 @@ union bpf_attr {
  *
  *		**-ENOENT** if the bpf_local_storage cannot be found.
  *
- * long bpf_d_path(struct path *path, char *buf, u32 sz)
+ * long bpf_d_path(const struct path *path, char *buf, u32 sz)
  *	Description
  *		Return full path for given **struct path** object, which
  *		needs to be the kernel BTF *path* object. The path is

tools/testing/selftests/bpf/progs/verifier_vfs_accept.c

@@ -70,7 +70,7 @@ __success
 int BPF_PROG(path_d_path_from_file_argument, struct file *file)
 {
 	int ret;
-	struct path *path;
+	const struct path *path;
 
 	/* The f_path member is a path which is embedded directly within a
 	 * file. Therefore, a pointer to such embedded members are still