md: Replace snprintf with scnprintf

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

md: Replace snprintf with scnprintf

Current code produces a warning as shown below when total characters
in the constituent block device names plus the slashes exceeds 200.
snprintf() returns the number of characters generated from the given
input, which could cause the expression “200 – len” to wrap around
to a large positive number. Fix this by using scnprintf() instead,
which returns the actual number of characters written into the buffer.

[ 1513.267938] ------------[ cut here ]------------
[ 1513.267943] WARNING: CPU: 15 PID: 37247 at <snip>/lib/vsprintf.c:2509 vsnprintf+0x2c8/0x510
[ 1513.267944] Modules linked in: <snip>
[ 1513.267969] CPU: 15 PID: 37247 Comm: mdadm Not tainted 5.4.0-1085-azure #90~18.04.1-Ubuntu
[ 1513.267969] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 05/09/2022
[ 1513.267971] RIP: 0010:vsnprintf+0x2c8/0x510
<-snip->
[ 1513.267982] Call Trace:
[ 1513.267986] snprintf+0x45/0x70
[ 1513.267990] ? disk_name+0x71/0xa0
[ 1513.267993] dump_zones+0x114/0x240 [raid0]
[ 1513.267996] ? _cond_resched+0x19/0x40
[ 1513.267998] raid0_run+0x19e/0x270 [raid0]
[ 1513.268000] md_run+0x5e0/0xc50
[ 1513.268003] ? security_capable+0x3f/0x60
[ 1513.268005] do_md_run+0x19/0x110
[ 1513.268006] md_ioctl+0x195e/0x1f90
[ 1513.268007] blkdev_ioctl+0x91f/0x9f0
[ 1513.268010] block_ioctl+0x3d/0x50
[ 1513.268012] do_vfs_ioctl+0xa9/0x640
[ 1513.268014] ? __fput+0x162/0x260
[ 1513.268016] ksys_ioctl+0x75/0x80
[ 1513.268017] __x64_sys_ioctl+0x1a/0x20
[ 1513.268019] do_syscall_64+0x5e/0x200
[ 1513.268021] entry_SYSCALL_64_after_hwframe+0x44/0xa9