Ensure that copy-on-writing a reflinked file when there's no free disk
space reflects the desired ENOSPC back to userspace during the write
call. Tests the buffered IO, direct IO, and mmap write paths.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>