disconnect takes too long time if there are many locks to cancel.
besides the amount of time spent on each lock cancel, there is a
resched() in cfs_hash_for_each_relax(), i.e. disconnect or eviction
may take unexpectedly long time. While this patch only contains
the client side fixes the original fix covered changes to both
the server and client code to ensure proper disconnect handling.
Below details the change done on both the server and client so
people can examine the disconnect behavior with both source bases.

- do not cancel locks on disconnect_export;
- export will be left in obd_unlinked_exports list pinned by live
  locks;
- new re-connects will created other non-conflicting exports;
- new locks will cancel obsolete locks on conflicts;
- once all the locks on the disconnected export will be cancelled,
  the export will be destroyed on the last ref put;
- do not cancel in small portions, cancel all together in just 1
  dedicated thread - use server side blocking thread for that;
- cancel blocked locks first so that waiting locks could proceed;
- take care about blocked waiting locks, so that they would get
  cancelled quickly too;
- do not remove lock from waiting list on AST error before moving
  it to elt_expired_locks list, because it removes it from export
  list too; otherwise this blocked lock will not be cancelled
  immediately on failed export;
- cancel lock instead of just destroy for failed export, to make
  full cleanup, i.e. remove it from export list.
Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3031
Xyratex-bug-id: MRP-395 MRP-1366 MRP-1366
Reviewed-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Alexey Lyashkov <Alexey_Lyashkov@xyratex.com>
Reviewed-on: http://review.whamcloud.com/5843Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>