RDMA/ucma: Remove unnecessary locking of file->ctx_list in close

During the file_operations release function it is already not possible
that write() can be running concurrently, remove the extra locking
around the ctx_list.

Link: https://lore.kernel.org/r/20200818120526.702120-3-leon@kernel.org


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