bpf: check_cond_jmp_op(): properly infer if register is null

In the Linux kernel, the following vulnerability has been resolved:

bpf: check_cond_jmp_op(): properly infer if register is null

Nicholas Carlini reported a bug when verifier can incorrectly infer
that a pointer is non-null. The bug occurs when two pointers are
compared and one of them has a type w/o PTR_MAYBE_NULL flag,
but which allows a value to be NULL at runtime.
Here is an example:

// `a` is PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED
// `a` is 0 at runtime.
// `b` is PTR_TO_MAP_VALUE | PTR_MAYBE_NULL
void *a = bpf_rdonly_cast(0, 0);
int *b = bpf_map_lookup_elem(...);

if (a == b)
*b = 42; // verifier does not catch null pointer dereference

This happens because of a special case in check_cond_jmp_op(),
which attempts to strip PTR_MAYBE_NULL flags from pointer types,
when processing comparisons like `rA == rB`, if either rA or rB can't
be null.

The non-null property is derived based on the absence of
PTR_MAYBE_NULL flag on rA's or rB's type. But that is not sufficient
for types like PTR_TO_MEM, as in the example.

This patch replaces type_may_be_null() call with reg_not_null(),
which contains an allowlist of types for which absence of
PTR_MAYBE_NULL actually means that the value can't be NULL at runtime.

At the moment, the list in the reg_not_null() omits two types for
which PTR_MAYBE_NULL is applicable: PTR_TO_XDP_SOCK and PTR_TO_BUF.
In order to remain backward compatible, and assuming that only
comparison between pointers of the same type makes sense,
this commit extends reg_not_null(). W/o such an extension e.g.
verifier_jeq_infer_not_null/null_ptr_to_map_value fails.

reg_not_null() can be extended further, but I deem that out of scope
for the fix at hand. Explicit base_type(...) != PTR_TO_BTF_ID
checks in the check_cond_jmp_op() can be removed with migration to
reg_not_null(), but that is a behavioural change, as the special case
would start matching for PTR_TO_BTF_ID that is also is_trusted_reg().
I omit the behavioural change from this commit.