bpf: fix precision bit propagation for BPF_ST instructions

When backtracking instructions to propagate precision bit for registers
and stack slots, one class of instructions (BPF_ST) weren't handled
causing extra stack slots to be propagated into parent state. Parent
state might not have that much stack allocated, though, which causes
warning on invalid stack slot usage.

This patch adds handling of BPF_ST instructions:

BPF_MEM | <size> | BPF_ST:   *(size *) (dst_reg + off) = imm32

Reported-by:  <syzbot+4da3ff23081bafe74fc2@syzkaller.appspotmail.com>
Fixes: b5dc0163 ("bpf: precise scalar_value tracking")
Cc: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>