Linux: bpf: Don't resurrect a scalar id dropped by collect_linked_regs()

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

bpf: Don't resurrect a scalar id dropped by collect_linked_regs()

check_cond_jmp_op() copies the compared registers into env->{false,true}_reg{1,2} before collect_linked_regs() runs and copies those snapshots back into both branch states afterwards.

collect_linked_regs() records at most LINKED_REGS_MAX members of a linked registers group in the jump history and calls clear_scalar_id() for every member that does not fit. The compared register is not exempt from that.

As a consequence, sync_linked_regs() might adjust ranges for more registers than bpf_bt_sync_linked_regs() can propagate precision to.

Collect the linked registers before the snapshots are taken instead. This might lead to some unnecessary clear_scalar_id's, but from previous testing situations with many linked registers are extremely rare.