xfstests: test speculative preallocation reclaim on ENOSPC/EDQUOT

xfstests: test speculative preallocation reclaim on ENOSPC/EDQUOT

XFS can allocate significant amounts of space to files via speculative
preallocation. Such preallocation may not be reclaimed automatically on
file close() if a file is repeatedly opened and extended. For smaller
filesystems with relatively large and slow growing files, this
preallocation can linger for some time, including contributing to out of
space conditions.

Create a situation where an fs is near out of space while several files
still have lingering, significant preallocations. Verify that new
writers reclaim the preallocated space rather than return ENOSPC. Repeat
a similar test for quota limits and EDQUOT.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>