Fix return value of make_filename() when no filename_format

Fix return value of make_filename() when no filename_format

This is unlikely to happen in the first place since ->filename_format
has a default value, and option parser can also detect an empty string,
but it should return buf which is sprintf'd right before returning.

A caller expects this function to return filename string (which is
buf arg itself in this case), and it also doesn't handle NULL return.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>