acpi: bus: check once more for an empty list after locking it

List could have become empty after the unlocked check that was made earlier,
so check again inside the lock.

Should fix https://bugzilla.redhat.com/show_bug.cgi?id=427765



Signed-off-by: Chuck Ebbert <cebbert@redhat.com>
Cc: <stable@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>