It closes mds sessions, drop all caps and invalidates page caches,
then use new entity address to reconnect to the cluster.

After reconnect, all dirty data/metadata are dropped, file locks
get lost sliently. Open files continue to work because client will
try renewing caps on later read/write.
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>