[SCSI] scsi_debug: add LBPRZ support

Add LBPRZ support to scsi_debug; i.e. read zeros for
unmapped blocks.

Rather than checking for unmapped blocks at
read time, this just zeroes them on the backing store
at unmap time so it behaves the same way.

This also adds a module parameter to disable it.

lbprz, "unmapped blocks return 0 on read (def=1)"

[jejb: fix whitespace errors]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>