bpf: fix check for attempt to corrupt spilled pointer

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

bpf: fix check for attempt to corrupt spilled pointer

When register is spilled onto a stack as a 1/2/4-byte register, we set

slot_type[BPF_REG_SIZE - 1] (plus potentially few more below it,

depending on actual spill size). So to check if some stack slot has

spilled register we need to consult slot_type[7], not slot_type[0].

To avoid the need to remember and double-check this in the future, just

use is_spilled_reg() helper.