md/raid1: Clean up local variable 'b' from raid1_read_request()

md/raid1: Clean up local variable 'b' from raid1_read_request()

The local variable will only be used onced, in the error path that
read_balance() failed to find a valid rdev to read. Since now the rdev
is ensured can't be removed from conf while IO is still pending,
remove the local variable and dereference rdev directly.

Since we're here, also remove an extra empty line, and unnecessary
type conversion from sector_t(u64) to unsigned long long.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240801133008.459998-1-yukuai1@huaweicloud.com
Signed-off-by: Song Liu <song@kernel.org>