ocfs2/dlm: Hold off sending lockres drop ref message while lockres is migrating

During lockres purge, o2dlm sends a drop reference message to the lockres
master. This patch delays the message if the lockres is being migrated.

Fixes oss bugzilla#1012
http://oss.oracle.com/bugzilla/show_bug.cgi?id=1012



Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>