Document the locking assumptions this function relies on and also verify
these locking assumptions at runtime.

Cc: Quinn Tran <qutran@marvell.com>
Cc: Daniel Wagner <dwagner@suse.de>
Link: https://lore.kernel.org/r/20200123042345.23886-2-bvanassche@acm.orgAcked-by: Himanshu Madhani <hmadhani@marvell.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>