Fix race in maintd start-up.
Fix race in maintd start-up.

When maintd initially starts, it must wait for lsd to inform it
which vnode/ctypes to maintain and then query adbd for information
about those namespaces (maintainer::init_ltree). If a sync request
comes in between these two phases, old code would deref NULL in
sync dispatch code; check for this case and reject RPCs until
the local tree has been initialized.

weidezhang/dht,weidezhang/dht,weidezhang/dht,weidezhang/dht,sit/dht,sit/dht,sit/dht,sit/dht,sit/dht,weidezhang/dht