scsi_debug: test always evaluates to false, || should be used instead

cppcheck found the following issue:
(warning) Logical conjunction always evaluates to false:
  alloc_len < 4 && alloc_len > 65535.

..the test should be instead:

  if (alloc_len < 4 || alloc_len > 65536)

This error was introduced by recent commit 38d5c833
("scsi_debug: add Report supported opcodes+tmfs; Compare and write")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>