Avoid ext4 auto_da_alloc flush on close (v2).

This updates commit 68640d56b2ea96401a1355ab56603b0837058d21.

Don't unlink the file: As Florian Weimer pointed out, this would cause
problems if someone uses '-o /dev/null' (it could delete /dev/null).

Don't write a byte.  The exact condition which causes the
auto_da_alloc misbehaviour is truncating a file from a large size to 0
size.  So this write was not necessary.

Thanks: Florian Weimer, Eric Sandeen, Eric Blake.