RAID: Improve an error message

When down-converting a RAID1 LV, if the user specifies too few devices,
they will get a confusing message.
Ex:
[root]# lvcreate -m 2 --type raid1 -n raid -L 500M taft
  Logical volume "raid" created

[root]# lvconvert -m 0 taft/raid /dev/sdd1
  Unable to extract enough images to satisfy request
  Failed to extract images from taft/raid

This patch makes the error message a bit clearer by telling the user
the count they are trying to remove and the number of devices they
supplied.

[root@bp-01 lvm2]# lvcreate --type raid1 -m 3 -L 200M -n lv vg
  Logical volume "lv" created

[root@bp-01 lvm2]# lvconvert -m -3 vg/lv /dev/sdb1
  Unable to remove 3 images:  Only 1 device given.
  Failed to extract images from vg/lv

[root@bp-01 lvm2]# lvconvert -m -3 vg/lv /dev/sd[bc]1
  Unable to remove 3 images:  Only 2 devices given.
  Failed to extract images from vg/lv

[root@bp-01 lvm2]# lvconvert -m -3 vg/lv /dev/sd[bcd]1
[root@bp-01 lvm2]# lvs -a -o name,attr,devices vg
  LV   Attr       Devices
  lv   -wi-a----- /dev/sde1(1)

This patch doesn't work in all cases.  The user can specify the right
number of devices, but not a sufficient amount of devices from the LV.
This will produce the old error message:
[root@bp-01 lvm2]# lvconvert -m -3 vg/lv /dev/sd[bcf]1
  Unable to extract enough images to satisfy request
  Failed to extract images from vg/lv
However, I think this error message is sufficient for this case.

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/metadata/raid_manip.c b/lib/metadata/raid_manip.c
index d1759aebf..f1cb01296 100644
--- a/lib/metadata/raid_manip.c
+++ b/lib/metadata/raid_manip.c
@@ -840,6 +840,12 @@ static int _raid_extract_images(struct logical_volume *lv, uint32_t new_count,
 	log_verbose("Extracting %u %s from %s/%s", extract,
 		    (extract > 1) ? "images" : "image",
 		    lv->vg->name, lv->name);
+	if (dm_list_size(target_pvs) < extract) {
+		log_error("Unable to remove %d images:  Only %d device%s given.",
+			  extract, dm_list_size(target_pvs),
+			  (dm_list_size(target_pvs) == 1) ? "" : "s");
+		return 0;
+	}
 
 	lvl_array = dm_pool_alloc(lv->vg->vgmem,
 				  sizeof(*lvl_array) * extract * 2);