BPF_STX instruction preserves STACK_ZERO marks for variable offset
writes in situations like below:

  *(u64*)(r10 - 8) = 0   ; STACK_ZERO marks for fp[-8]
  r0 = random(-7, -1)    ; some random number in range of [-7, -1]
  r0 += r10              ; r0 is now a variable offset pointer to stack
  r1 = 0
  *(u8*)(r0) = r1        ; BPF_STX writing zero, STACK_ZERO mark for
                         ; fp[-8] is preserved

This commit updates verifier.c:check_stack_write_var_off() to process
BPF_ST in a similar manner, e.g. the following example:

  *(u64*)(r10 - 8) = 0   ; STACK_ZERO marks for fp[-8]
  r0 = random(-7, -1)    ; some random number in range of [-7, -1]
  r0 += r10              ; r0 is now variable offset pointer to stack
  *(u8*)(r0) = 0         ; BPF_ST writing zero, STACK_ZERO mark for
                         ; fp[-8] is preserved
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230214232030.1502829-4-eddyz87@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>