rbd: create pool_id device attribute

Add an entry under /sys/bus/rbd/devices/<N>/ named "pool_id" that
provides the id for the pool the rbd image is assocatied with.  This
is in addition to the pool name already provided.

Rename the "poolid" field in struct rbd_device  to be "pool_id".

Update the documentation to reflect the addition of this new entry.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/Documentation/ABI/testing/sysfs-bus-rbd b/Documentation/ABI/testing/sysfs-bus-rbd
index bcd88eb..3c17b62 100644
--- a/Documentation/ABI/testing/sysfs-bus-rbd
+++ b/Documentation/ABI/testing/sysfs-bus-rbd
@@ -35,8 +35,14 @@
 
 pool
 
-	The pool where this rbd image resides. The pool-name pair is unique
-	per rados system.
+	The name of the storage pool where this rbd image resides.
+	An rbd image name is unique within its pool.
+
+pool_id
+
+	The unique identifier for the rbd image's pool.  This is
+	a permanent attribute of the pool.  A pool's id will never
+	change.
 
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