riscv: signal: fix signal frame size

riscv: signal: fix signal frame size

The signal context of certain RISC-V extensions will be appended after
struct __riscv_extra_ext_header, which already includes an empty context
header. Therefore, there is no need to preserve a separate hdr for the
END of signal context.

Fixes: 8ee0b41898fa ("riscv: signal: Add sigcontext save/restore for vector")
Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Andy Chiu <AndybnAC@gmail.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20241220083926.19453-2-yongxuan.wang@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>