bpf: mark PTR_TO_MEM as non-null register type

PTR_TO_MEM register without PTR_MAYBE_NULL is indeed non-null. This is
important for BPF verifier to be able to prune guaranteed not to be
taken branches. This is always the case with open-coded iterators.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230302235015.2044271-11-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -487,7 +487,8 @@ static bool reg_type_not_null(enum bpf_reg_type type)
 		type == PTR_TO_TCP_SOCK ||
 		type == PTR_TO_MAP_VALUE ||
 		type == PTR_TO_MAP_KEY ||
-		type == PTR_TO_SOCK_COMMON;
+		type == PTR_TO_SOCK_COMMON ||
+		type == PTR_TO_MEM;
 }
 
 static bool type_is_ptr_alloc_obj(u32 type)