IB/uverbs: Release event file reference on ib_uverbs_create_cq() error

ib_uverbs_create_cq() should release the completion channel event file
if an error occurs after it looks it up.  Also, if userspace asks for
a completion channel and we don't find it, an error should be returned
instead of silently creating a CQ without a completion channel.

Signed-off-by: Jack Morgenstein <jackm@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>