RDMA/core: Allow detaching gid attribute netdevice for RoCE

When there is active traffic through a GID, a QP/AH holds reference to
this GID entry. RoCE GID entry holds reference to its attached
netdevice. Due to this when netdevice is deleted by admin user, its
refcount is not dropped.

Therefore, while deleting RoCE GID, wait for all GID attribute's netdev
users to finish accessing netdev in rcu context.  Once all users done
accessing it, release the netdev refcount.

Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>