xfs: test for premature ENOSPC with large cow delalloc extents

xfs: test for premature ENOSPC with large cow delalloc extents

On a higly fragmented filesystem a Direct IO write can fail with -ENOSPC error
even though the filesystem has sufficient number of free blocks.

This occurs if the file offset range on which the write operation is being
performed has a delalloc extent in the cow fork and this delalloc extent
begins much before the Direct IO range.

In such a scenario, xfs_reflink_allocate_cow() invokes xfs_bmapi_write() to
allocate the blocks mapped by the delalloc extent. The extent thus allocated
may not cover the beginning of file offset range on which the Direct IO write
was issued. Hence xfs_reflink_allocate_cow() ends up returning -ENOSPC.

This test addresses this issue.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>