relay: move remove_buf_file inside relay_close_buf

relay: move remove_buf_file inside relay_close_buf

Currently remove_buf_file callback is called from from kobject
release method. This result in follow issue:
# blktrace -d /dev/sda1 -d /dev/sda -o test

blktrace_setup()
 dir = create_dir()
 rchan = relay_open(dir,...)
 ->create_buf_file_callback
    buf_file  = debugfs_create_file(dir, )

Userspace will open buf_file.
Later we make a decision to stop tracing
blktrace_down()
  relay_close(rhcan)  /* just decrement kobj reference  */
                      /* since it is not zero then callback not called */
  debugfs_remove(dir) /* FAIL due to non empty dir   */

Later user space will close the file and file will be deleted,
but directory still exist.
user_space_close()
 ->file_release
   ->release_buf_file_callback
     ->debugfs_remove(buf_file
## TESTCASE:
# blktrace -d /dev/sda1 -d /dev/sda -o test
# After that blktrace infrastructure will remain broken in
# an unusable state so: blktrace -d /dev/sda1 will not work.

In fact this is general issue, blktrace is just one of examples.
We can not reliably remove parent dir until all users close the
buf_file.

Solution: We don't have to wait that long. File should be deleted inside
relay_close_buf().

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>