With strange workloads which do a lot of quick truncation on small filesystems
it is possible to get into a situation where there are free blocks on the
disk, but they are not allocatable at this time due to their having been freed
up in the current JBD transaction.  Applications get unexpected ENOSPC errors.

We can fix that with this patch, originally by Andreas Dilger which forces a
single commit+retry when an ENOSPC is encountered.