RDMA/ucma: Fix locking for ctx->events_reported

RDMA/ucma: Fix locking for ctx->events_reported

This value is locked under the file->mut, ensure it is held whenever
touching it.

The case in ucma_migrate_id() is a race, while in ucma_free_uctx() it is
already not possible for the write side to run, the movement is just for
clarity.

Fixes: 31d9d9a4fd72 ("RDMA/cma: add support for rdma_migrate_id()")
Link: https://lore.kernel.org/r/20200818120526.702120-10-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>