IB/ipath: Really run work in ipath_release_user_pages_on_close()

ipath_release_user_pages_on_close() just allocated a structure to
schedule work with but just returned (leaking the structure) rather than 
actually doing schedule_work().  Fix the logic to what was intended.

This was spotted by the Coverity checker (CID 2700).

Signed-off-by: Roland Dreier <rolandd@cisco.com>