There's a KASAN warning in raid5_remove_disk when running the LVM
testsuite. We fix this warning by verifying that the "number" variable is
within limits.

Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>