cpu/hotplug: Add debug printks for hotplug callback failures

CPU hotplug callbacks can fail and cause a rollback to the previous
state. These failures are silent and therefore hard to debug.

Add pr_debug() to the up and down paths which provide information about the
error code, the CPU and the failed state. The debug printks can be enabled
via kernel command line or sysfs.

[ tglx: Adopt to current mainline, massage printk and changelog ]

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Qais Yousef <qais.yousef@arm.com>
Link: https://lore.kernel.org/r/20210409055316.1709-1-dongli.zhang@oracle.com