scsi: zfcp: Use prandom_u32_max() for backoff

scsi: zfcp: Use prandom_u32_max() for backoff

We don't need crypto-grade random numbers for randomized backoffs.  Instead
use prandom_u32_max(ep_ro) which generates a pseudo-random number uniformly
distributed in the interval [0, ep_ro).

Link: https://lore.kernel.org/r/8fc7c4c4069ff1783f4a9ccd84a923f581a09ec5.1593780621.git.bblock@linux.ibm.com
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: George Spelvin <lkml@sdf.org>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>