bpf: Fix bpf_xdp_store_bytes proto for read-only arg

While making some maps in Cilium read-only from the BPF side, we noticed
that the bpf_xdp_store_bytes proto is incorrect. In particular, the
verifier was throwing the following error:

; ret = ctx_store_bytes(ctx, l3_off + offsetof(struct iphdr, saddr),
&nat->address, 4, 0);
635: (79) r1 = *(u64 *)(r10 -144) ; R1=ctx() R10=fp0 fp-144=ctx()
636: (b4) w2 = 26 ; R2=26
637: (b4) w4 = 4 ; R4=4
638: (b4) w5 = 0 ; R5=0
639: (85) call bpf_xdp_store_bytes#190
write into map forbidden, value_size=6 off=0 size=4

nat comes from a BPF_F_RDONLY_PROG map, so R3 is a PTR_TO_MAP_VALUE.
The verifier checks the helper's memory access to R3 in
check_mem_size_reg, as it reaches ARG_CONST_SIZE argument. The third
argument has expected type ARG_PTR_TO_UNINIT_MEM, which includes the
MEM_WRITE flag. The verifier thus checks for a BPF_WRITE access on R3.
Given R3 points to a read-only map, the check fails.

Conversely, ARG_PTR_TO_UNINIT_MEM can also lead to the helper reading
from uninitialized memory.

This patch simply fixes the expected argument type to match that of
bpf_skb_store_bytes.

Fixes: 3f364222d032 ("net: xdp: introduce bpf_xdp_pointer utility routine")
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/9fa3c9f72d806e82541071c4df88b8cba28ad6a9.1769875479.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>