bpf: Fix linked reg delta tracking when src_reg == dst_reg

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

bpf: Fix linked reg delta tracking when src_reg == dst_reg

Consider the case of rX += rX where src_reg and dst_reg are pointers to the same bpf_reg_state in adjust_reg_min_max_vals(). The latter first modifies the dst_reg in-place, and later in the delta tracking, the subsequent is_reg_const(src_reg)/reg_const_value(src_reg) reads the post-{add,sub} value instead of the original source.

This is problematic since it sets an incorrect delta, which sync_linked_regs() then propagates to linked registers, thus creating a verifier-vs-runtime mismatch. Fix it by just skipping this corner case.