xfsqa: add delayed allocation @ ENOSPC exerciser

xfsqa: add delayed allocation @ ENOSPC exerciser

When we deplete the reserve block pool or receive an error during
delayed allocation, we currently toss the page away. If that page
has delayed allocation extents on it, we can fail to remove them and
leave stale delayed llocation extents lying around on the inode.
This can, in certain circumstances, trigger errors later on when the
stale delalloc extent it found again, including tripping a BUG().

Exercise this failure path so that we get code coverage of the fix
that prevents stale delalloc mappings from being left on the inode
when pages are tossed.

This is based on a test case supplied by Lachlan McIlroy.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>