md/raid1: factor out helper to handle blocked rdev from raid1_write_request()

md/raid1: factor out helper to handle blocked rdev from raid1_write_request()

Currently raid1 is preparing IO for underlying disks while checking if
any disk is blocked, if so allocated resources must be released, then
waiting for rdev to be unblocked and try to prepare IO again.

Make code cleaner by checking blocked rdev first, it doesn't matter if
rdev is blocked while issuing IO, the IO will wait for rdev to be
unblocked or not.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Tested-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Link: https://lore.kernel.org/r/20241031033114.3845582-5-yukuai1@huaweicloud.com
Signed-off-by: Song Liu <song@kernel.org>