IB/uverbs: Use alloc_file() instead of get_empty_filp()

Christoph Hellwig wants to unexport get_empty_filp(), which is an ugly
internal interface.  Change the modular user in ib_uverbs_alloc_event_file()
to use the better alloc_file() interface; this makes the code cleaner too.

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