workqueue: fix -Wformat-truncation in create_worker

Compiling with W=1 emitted the following warning
(Compiler: gcc (x86-64, ver. 13.2.1, .config: result of make allyesconfig,
"Treat warnings as errors" turned off):

kernel/workqueue.c:2188:54: warning: ‘%d’ directive output may be
	truncated writing between 1 and 10 bytes into a region of size
	between 5 and 14 [-Wformat-truncation=]
kernel/workqueue.c:2188:50: note: directive argument in the range
	[0, 2147483647]
kernel/workqueue.c:2188:17: note: ‘snprintf’ output between 4 and 23 bytes
	into a destination of size 16

setting "id_buf" to size 23 will silence the warning, since GCC
determines snprintf's output to be max. 23 bytes in line 2188.

Please let me know if there are any mistakes in my patch!

Signed-off-by: Lucy Mielke <lucymielke@icloud.com>
Signed-off-by: Tejun Heo <tj@kernel.org>