acpi_memhotplug.c: don't allow to eject the memory device if it is being used

acpi_memhotplug.c: don't allow to eject the memory device if it is being used

We eject the memory device even if it is in use.  It is very dangerous,
and it will cause the kernel to panic.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Cc: Len Brown <lenb@kernel.org>
Cc: "Brown, Len" <len.brown@intel.com>
Cc: Yasuaki ISIMATU <isimatu.yasuaki@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>