scsi: scsi_dh_alua: Avoid crash during alua_bus_detach()

alua_bus_detach() might be running concurrently with alua_rtpg_work(), so
we might trip over h->sdev == NULL and call BUG_ON().  The correct way of
handling it is to not set h->sdev to NULL in alua_bus_detach(), and call
rcu_synchronize() before the final delete to ensure that all concurrent
threads have left the critical section.  Then we can get rid of the
BUG_ON() and replace it with a simple if condition.

Link: https://lore.kernel.org/r/1600167537-12509-1-git-send-email-jitendra.khasdev@oracle.com
Link: https://lore.kernel.org/r/20200924104559.26753-1-hare@suse.de


Cc: Brian Bunker <brian@purestorage.com>
Acked-by: Brian Bunker <brian@purestorage.com>
Tested-by: Jitendra Khasdev <jitendra.khasdev@oracle.com>
Reviewed-by: Jitendra Khasdev <jitendra.khasdev@oracle.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>