IB/ipoib: Set IPOIB_NEIGH_TBL_FLUSH after flushed completion initialization

Set IPOIB_NEIGH_TBL_FLUSH bit after initializing the neighbor
flushed completion, otherwise the garbage collector may signal
a completion while it is not initialized yet.

Fixes: b63b70d8 ("IPoIB: Use a private hash table for path lookup in xmit path")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>