md: fix kmemleak of rdev->serial

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

md: fix kmemleak of rdev->serial

If kobject_add() is fail in bind_rdev_to_array(), 'rdev->serial' will be

alloc not be freed, and kmemleak occurs.

unreferenced object 0xffff88815a350000 (size 49152):

comm "mdadm", pid 789, jiffies 4294716910

hex dump (first 32 bytes):

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

backtrace (crc f773277a):

[<0000000058b0a453>] kmemleak_alloc+0x61/0xe0

[<00000000366adf14>] __kmalloc_large_node+0x15e/0x270

[<000000002e82961b>] __kmalloc_node.cold+0x11/0x7f

[<00000000f206d60a>] kvmalloc_node+0x74/0x150

[<0000000034bf3363>] rdev_init_serial+0x67/0x170

[<0000000010e08fe9>] mddev_create_serial_pool+0x62/0x220

[<00000000c3837bf0>] bind_rdev_to_array+0x2af/0x630

[<0000000073c28560>] md_add_new_disk+0x400/0x9f0

[<00000000770e30ff>] md_ioctl+0x15bf/0x1c10

[<000000006cfab718>] blkdev_ioctl+0x191/0x3f0

[<0000000085086a11>] vfs_ioctl+0x22/0x60

[<0000000018b656fe>] __x64_sys_ioctl+0xba/0xe0

[<00000000e54e675e>] do_syscall_64+0x71/0x150

[<000000008b0ad622>] entry_SYSCALL_64_after_hwframe+0x6c/0x74