target/rd: fix or rewrite the copy routine

So the code assumes that the sg list is only a array while in reality
loopback SGL memory via scsi_cmnd into target-core may be already
chained.  This patch converts ramdisk code to use sg_miter logic from
scatterlist.h in order to properly support passthrough SGL usage with
transport_generic_map_mem_to_cmd() via loopback.

With this patch the bug goes away. However after umount/mount of the
device my files are gone. So something is still not right. After looking
at it for a while I decided to rewrite the that part of the code and now
things do work for me.

For reference:
- http://article.gmane.org/gmane.linux.scsi.target.devel/595
  the sg_next() conversion
- http://article.gmane.org/gmane.linux.scsi.target.devel/602


  the rewrite of the copy code

(nab: Fix compile warning in rd_MEMCPY)

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>