RDMA/core: Remove ucontext->closing

Nothing reads this any more, and the reason for its existence has passed
due to the deferred fput() scheme.

Fixes: 8ea1f989

 ("drivers/IB,usnic: reduce scope of mmap_sem")
Link: https://lore.kernel.org/r/0-v1-df64ff042436+42-uctx_closing_jgg@nvidia.com
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>