ceph: add helper function that forcibly reconnects to ceph cluster.

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>