IB/core: Rename uverbs event file structure

Previously, ib_uverbs_event_file was suffixed by _file as it contained
the actual file information. Since it's now only used as base struct
for ib_uverbs_async_event_file and ib_uverbs_completion_event_file,
we change its name to ib_uverbs_event_queue. This represents its
logical role better.

Fixes: 1e7710f3 ('IB/core: Change completion channel to use the reworked objects schema')
Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>