xen/blback: Fit the important information of the thread in 17 characters

The processes names are truncated to 17, while we had the length
of the process as name 20 - which meant that while we filled
it out with various details - the last 3 characters (which had
the queue number) never surfaced to the user-space.

To simplify this and be able to fit the device name, domain id,
and the queue number we remove the 'blkback' from the name.

Prior to this patch the device name is "blkback.<domid>.<name>"
for example: blkback.8.xvda, blkback.11.hda.

With the multiqueue block backend we add "-%d" for the queue.
But sadly this is already way past the limit so it gets stripped.

Possible solution had been identified by Ian:
http://lists.xenproject.org/archives/html/xen-devel/2015-05/msg03516.html

  "
  If you are pressed for space then the "xvd" is probably a bit redundant
  in a string which starts blkbk.

  The guest may not even call the device xvdN (iirc BSD has another
  prefix) any how, so having blkback say so seems of limited use anyway.

  Since this seems to not include a partition number how does this work in
  the split partition scheme? (i.e. one where the guest is given xvda1 and
  xvda2 rather than xvda with a partition table)

[It will be 'blkback.8.xvda1', and 'blkback.11.xvda2']

  Perhaps something derived from one of the schemes in
  http://xenbits.xen.org/docs/unstable/misc/vbd-interface.txt might be a
  better fit?

After a bit of discussion (see
http://lists.xenproject.org/archives/html/xen-devel/2015-12/msg01588.html)
we settled on dropping the "blback" part.

This will make it possible to have the <domid>.<name>-<queue>:

 [1.xvda-0]
 [1.xvda-1]

And we enough space to make it go up to:

 [32100.xvdfg9-5]
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/xenbus.c

@@ -23,8 +23,7 @@
 #include <xen/grant_table.h>
 #include "common.h"
 
-/* Enlarge the array size in order to fully show blkback name. */
-#define BLKBACK_NAME_LEN (20)
+/* On the XenBus the max length of 'ring-ref%u'. */
 #define RINGREF_NAME_LEN (20)
 
 struct backend_info {
@@ -76,7 +75,7 @@ static int blkback_name(struct xen_blkif *blkif, char *buf)
 	else
 		devname  = devpath;
 
-	snprintf(buf, BLKBACK_NAME_LEN, "blkback.%d.%s", blkif->domid, devname);
+	snprintf(buf, TASK_COMM_LEN, "%d.%s", blkif->domid, devname);
 	kfree(devpath);
 
 	return 0;
@@ -85,7 +84,7 @@ static int blkback_name(struct xen_blkif *blkif, char *buf)
 static void xen_update_blkif_status(struct xen_blkif *blkif)
 {
 	int err;
-	char name[BLKBACK_NAME_LEN];
+	char name[TASK_COMM_LEN];
 	struct xen_blkif_ring *ring;
 	int i;