RDMA/ucma: Narrow file->mut in ucma_event_handler()

Since the backlog is now an atomic the file->mut is now only protecting
the event_list and ctx_list. Narrow its scope to make it clear

Link: https://lore.kernel.org/r/20200818120526.702120-13-leon@kernel.org


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