EXTENDED_COPY needs to be able to search a global list of devices
based on NAA WWN device identifiers, so add a simple g_device_list
protected by g_device_mutex.
Cc: Christoph Hellwig <hch@lst.de> Cc: Hannes Reinecke <hare@suse.de> Cc: Martin Petersen <martin.petersen@oracle.com> Cc: Chris Mason <chris.mason@fusionio.com> Cc: Roland Dreier <roland@purestorage.com> Cc: Zach Brown <zab@redhat.com> Cc: James Bottomley <JBottomley@Parallels.com> Cc: Nicholas Bellinger <nab@linux-iscsi.org> Signed-off-by: Nicholas Bellinger <nab@daterainc.com>