test/file_locks: use struct flock instead of struct flock64

If we want to use flock64, we have to use fcntl64. Otherwise we will get
errors like this:
ERR: file_locks07.c:49: Can't set ofd lock (errno = 22 (Invalid argument))

Reported-by: Mr Jenkins
Signed-off-by: Andrei Vagin <avagin@gmail.com>