ext4: Don't treat a truncation of a zero-length file as replace-via-truncate

If a non-existent file is opened via O_WRONLY|O_CREAT|O_TRUNC, there's
no need to treat this as a true file truncation, so we shouldn't
activate the replace-via-truncate hueristic.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>