RDMA/umem: Restore lockdep check while downgrading lock

Lockdep engine handles correctly downgrade of locks and it simply
incorrect to disable lockdep checks prior to calling mmu_notifier.

Remove lockdep_off and ensure locks correctness.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>