scsi: scsi_debug: add cmd abort option to every_nth

This patch is motivated by a response in the thread:

  Re: [PATCH 0/5]stop normal completion path entering a timeout req

by Jianchao Wang . It generalizes the error injection of
blk_abort_request() to use scsi_debug's "every_nth" mechanism.  Ref with
original patch to scsi_debug:

https://lore.kernel.org/lkml/a68ad043-26a1-d3d8-2009-504ba4230e0f@oracle.com/



Also convert two vmalloc/memset(0) to vzalloc() calls.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>