bpf: Reject invalid LDSX instruction in disassembly

The signed-load mnemonic table has entries for byte, half-word, and word
loads because BPF_MEMSX does not support double-word loads. A BPF_MEMSX
| BPF_DW instruction nevertheless selects index 3, past the end of this
table.

Program Structure diagnostics can disassemble a malformed instruction
before check_and_resolve_insns() rejects its opcode. Placing the invalid
signed double-word load at the end of a program therefore triggers an
out-of-bounds access while reporting subprogram fallthrough.

Treat signed double-word loads as invalid in the disassembler and use
the existing BUG_ldx fallback instead.

Fixes: a8f427835394 ("bpf: Report Program Structure CFG errors")
Reported-by: syzbot+3544d9b2a9206be8ba37@syzkaller.appspotmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://lore.kernel.org/bpf/20260820022020.3450479-2-memxor@gmail.com

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