Fix arc_adapt() spinning in iterate_supers_type()

The iterate_supers_type() function which was introduced in the
3.0 kernel was supposed to provide a safe way to call an arbitrary
function on all super blocks of a specific type.  Unfortunately,
because a list_head was used a bug was introduced which made it
possible for iterate_supers_type() to get stuck spinning on a
super block which was just deactivated.

This can occur because when the list head is removed from the
fs_supers list it is reinitialized to point to itself.  If the
iterate_supers_type() function happened to be processing the
removed list_head it will get stuck spinning on that list_head.

The bug was fixed in the 3.3 kernel by converting the list_head
to an hlist_node.  However, to resolve the issue for existing
3.0 - 3.2 kernels we detect when a list_head is used.  Then to
prevent the spinning from occurring the .next pointer is set to
the fs_supers list_head which ensures the iterate_supers_type()
function will always terminate.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1045
Closes #861
Closes #790

module/zfs/zpl_super.c

@@ -240,6 +240,10 @@ zpl_kill_sb(struct super_block *sb)
 {
 	zfs_preumount(sb);
 	kill_anon_super(sb);
+
+#ifdef HAVE_S_INSTANCES_LIST_HEAD
+	sb->s_instances.next = &(zpl_fs_type.fs_supers);
+#endif /* HAVE_S_INSTANCES_LIST_HEAD */
 }
 
 #ifdef HAVE_SHRINK