RISC-V: KVM: fix stack overrun when loading vlenb

RISC-V: KVM: fix stack overrun when loading vlenb

The userspace load can put up to 2048 bits into an xlen bit stack
buffer.  We want only xlen bits, so check the size beforehand.

Fixes: 2fa290372dfe ("RISC-V: KVM: add 'vlenb' Vector CSR")
Cc: stable@vger.kernel.org
Signed-off-by: Radim Krčmář <rkrcmar@ventanamicro.com>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Link: https://lore.kernel.org/r/20250805104418.196023-4-rkrcmar@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>