IB/uverbs: Don't store struct file * for event files

The file member of struct ib_uverbs_event_file was only used to keep
track of whether the file had been closed or not.  The only thing we
ever did with the value was check if it was NULL or not.  Simplify the
code and get rid of the need to keep track of the struct file * we
allocate by replacing the file member with an is_closed member.

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