ocfs2/cluster: Add new function o2net_fill_node_map()

Patch adds function o2net_fill_node_map() to return the bitmap of nodes that
it is connected to. This bitmap is also accessible by the user via the debugfs
file, /sys/kernel/debug/o2net/connected_nodes.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>