md: raid1: check rdev before reference in raid1_sync_request func

[ Upstream commit 028288df635f5a9addd48ac4677b720192747944 ]

In raid1_sync_request func, rdev should be checked before reference.

Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>