[S390] xpram: per device block request queues.

The xpram driver uses a single block device queue for all of its
devices so far. With recent kernels removing xpram module fails to
clean up all sysfs files. The next time the xpram module is loaded
you'll get warnings:

  WARNING: at fs/sysfs/dir.c:463 sysfs_add_one+0x5e/0x64()
  sysfs: duplicate filename '35:0' can not be created
  Modules linked in: xpram(+) [last unloaded: xpram]

Followed by the usual WARN_ON output, followed by an error message
from kobject_add_internal, followed by a badness in genhd. Allocating
a block queue per device fixes this.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>