Implement flock inode operation to support advisory file locking.  All
lock/unlock operations are synchronous with the MDS.  Lock state is
sent when reconnecting to a recovering MDS to restore the shared lock
state.
Signed-off-by: NGreg Farnum <gregf@hq.newdream.net>
Signed-off-by: NSage Weil <sage@newdream.net>