LoongArch: Fix MAX_REG_OFFSET calculation

commit 90436d23 upstream.

Fix MAX_REG_OFFSET calculation, make it point to the last register
in 'struct pt_regs' and not to the marker itself, which could allow
regs_get_register() to return an invalid offset.

Cc: stable@vger.kernel.org
Fixes: 803b0fc5 ("LoongArch: Add process management")
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>