IB/uverbs: Pack struct ib_uverbs_event_file tighter

Eliminate some padding in the structure by rearranging the members.
sizeof(struct ib_uverbs_event_file) is now 72 bytes (from 80) and
more members now fit in the first cacheline.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>