md: raid1: fix potential OOB in raid1_remove_disk()

In the Linux kernel, the following vulnerability has been resolved:

md: raid1: fix potential OOB in raid1_remove_disk()

If rddev->raid_disk is greater than mddev->raid_disks, there will be

an out-of-bounds in raid1_remove_disk(). We have already found

similar reports as follows:

1) commit d17f744e883b ("md-raid10: fix KASAN warning")

2) commit 1ebc2cec0b7d ("dm raid: fix KASAN warning in raid5_remove_disk")

Fix this bug by checking whether the "number" variable is

valid.