s390/pci: Serialize device addition and removal

Prior changes ensured that when zpci_release_device() is called and it
removed the zdev from the zpci_list this instance can not be found via
the zpci_list anymore even while allowing re-add of reserved devices.
This only accounts for the overall lifetime and zpci_list addition and
removal, it does not yet prevent concurrent add of a new instance for
the same underlying device. Such concurrent add would subsequently cause
issues such as attempted re-use of the same IOMMU sysfs directory and is
generally undesired.

Introduce a new zpci_add_remove_lock mutex to serialize adding a new
device with removal. Together this ensures that if a struct zpci_dev is
not found in the zpci_list it was either already removed and torn down,
or its removal and tear down is in progress with the
zpci_add_remove_lock held.

Cc: stable@vger.kernel.org
Fixes: a46044a9 ("s390/pci: fix zpci_zdev_put() on reserve")
Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Tested-by: Gerd Bayer <gbayer@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>