ext4: Automatically allocate delay allocated blocks on close

When closing a file that had been previously truncated, force any
delay allocated blocks that to be allocated so that if the filesystem
is mounted with data=ordered, the data blocks will be pushed out to
disk along with the journal commit.  Many application programs expect
this, so we do this to avoid zero length files if the system crashes
unexpectedly.

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