bpf: Handle BPF_UPTR in verifier

bpf: Handle BPF_UPTR in verifier

This patch adds BPF_UPTR support to the verifier. Not that only the
map_value will support the "__uptr" type tag.

This patch enforces only BPF_LDX is allowed to the value of an uptr.
After BPF_LDX, it will mark the dst_reg as PTR_TO_MEM | PTR_MAYBE_NULL
with size deduced from the field.kptr.btf_id. This will make the
dst_reg pointed memory to be readable and writable as scalar.

There is a redundant "val_reg = reg_state(env, value_regno);" statement
in the check_map_kptr_access(). This patch takes this chance to remove
it also.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20241023234759.860539-3-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>