bpf: Replace the document for PTR_TO_BTF_ID_OR_NULL

Commit c25b2ae ("bpf: Replace PTR_TO_XXX_OR_NULL with PTR_TO_XXX |
PTR_MAYBE_NULL") moved the fields around and misplaced the
documentation for "PTR_TO_BTF_ID_OR_NULL". So, let's replace it in the
proper place.

Signed-off-by: Menglong Dong <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: image-dragon

include/linux/bpf.h

@@ -932,10 +932,6 @@ enum bpf_reg_type {
 	 * additional context, assume the value is non-null.
 	 */
 	PTR_TO_BTF_ID,
-	/* PTR_TO_BTF_ID_OR_NULL points to a kernel struct that has not
-	 * been checked for null. Used primarily to inform the verifier
-	 * an explicit null check is required for this struct.
-	 */
 	PTR_TO_MEM,		 /* reg points to valid memory region */
 	PTR_TO_ARENA,
 	PTR_TO_BUF,		 /* reg points to a read/write buffer */
@@ -948,6 +944,10 @@ enum bpf_reg_type {
 	PTR_TO_SOCKET_OR_NULL		= PTR_MAYBE_NULL | PTR_TO_SOCKET,
 	PTR_TO_SOCK_COMMON_OR_NULL	= PTR_MAYBE_NULL | PTR_TO_SOCK_COMMON,
 	PTR_TO_TCP_SOCK_OR_NULL		= PTR_MAYBE_NULL | PTR_TO_TCP_SOCK,
+	/* PTR_TO_BTF_ID_OR_NULL points to a kernel struct that has not
+	 * been checked for null. Used primarily to inform the verifier
+	 * an explicit null check is required for this struct.
+	 */
 	PTR_TO_BTF_ID_OR_NULL		= PTR_MAYBE_NULL | PTR_TO_BTF_ID,
 
 	/* This must be the last entry. Its purpose is to ensure the enum is