bpf: Tell bpf programs kernel's PAGE_SIZE

vmlinux BTF includes all kernel enums.
Add __PAGE_SIZE = PAGE_SIZE enum, so that bpf programs
that include vmlinux.h can easily access it.
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20240307031228.42896-7-alexei.starovoitov@gmail.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/core.c

@@ -88,13 +88,18 @@ void *bpf_internal_load_pointer_neg_helper(const struct sk_buff *skb, int k, uns
 	return NULL;
 }
 
+/* tell bpf programs that include vmlinux.h kernel's PAGE_SIZE */
+enum page_size_enum {
+	__PAGE_SIZE = PAGE_SIZE
+};
+
 struct bpf_prog *bpf_prog_alloc_no_stats(unsigned int size, gfp_t gfp_extra_flags)
 {
 	gfp_t gfp_flags = bpf_memcg_flags(GFP_KERNEL | __GFP_ZERO | gfp_extra_flags);
 	struct bpf_prog_aux *aux;
 	struct bpf_prog *fp;
 
-	size = round_up(size, PAGE_SIZE);
+	size = round_up(size, __PAGE_SIZE);
 	fp = __vmalloc(size, gfp_flags);
 	if (fp == NULL)
 		return NULL;