selftests/bpf: Fix the u64_offset_to_skb_data test

The u64_offset_to_skb_data test is supposed to make a 64-bit fill, but
instead makes a 16-bit one. Fix the test according to its intention and
update the comments accordingly (umax is no longer 0xffff). The 16-bit
fill is covered by u16_offset_to_skb_data.
Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240108205209.838365-2-maxtram95@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -243,7 +243,7 @@ l0_%=:	r0 = 0;						\
 
 SEC("tc")
 __description("Spill u32 const scalars.  Refill as u64.  Offset to skb->data")
-__failure __msg("invalid access to packet")
+__failure __msg("math between pkt pointer and register with unbounded min value is not allowed")
 __naked void u64_offset_to_skb_data(void)
 {
 	asm volatile ("					\
@@ -253,13 +253,11 @@ __naked void u64_offset_to_skb_data(void)
 	w7 = 20;					\
 	*(u32*)(r10 - 4) = r6;				\
 	*(u32*)(r10 - 8) = r7;				\
-	r4 = *(u16*)(r10 - 8);				\
+	r4 = *(u64*)(r10 - 8);				\
 	r0 = r2;					\
-	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4=umax=65535 */\
+	/* r0 += r4 R0=pkt R2=pkt R3=pkt_end R4= */	\
 	r0 += r4;					\
-	/* if (r0 > r3) R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=umax=65535 */\
 	if r0 > r3 goto l0_%=;				\
-	/* r0 = *(u32 *)r2 R0=pkt,umax=65535 R2=pkt R3=pkt_end R4=20 */\
 	r0 = *(u32*)(r2 + 0);				\
 l0_%=:	r0 = 0;						\
 	exit;						\