bpf: Fix an array-index-out-of-bounds issue in disasm.c

syzbot reported an array-index-out-of-bounds when printing out bpf
insns. Further investigation shows the insn is illegal but
is printed out due to log level 1 or 2 before actual insn verification
in do_check().

This particular illegal insn is a MOVSX insn with offset value 2.
The legal offset value for MOVSX should be 8, 16 and 32.
The disasm sign-extension-size array index is calculated as
 (insn->off / 8) - 1
and offset value 2 gives an out-of-bound index -1.

Tighten the checking for MOVSX insn in disasm.c to avoid
array-index-out-of-bounds issue.

Reported-by:  <syzbot+3758842a6c01012aa73b@syzkaller.appspotmail.com>
Fixes: f835bb62 ("bpf: Add kernel/bpftool asm support for new instructions")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230731204534.1975311-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>