reflink: ensure that we can handle reflinking a lot of extents

reflink: ensure that we can handle reflinking a lot of extents

Update the existing stress tests to ensure that we can handle
reflinking the same block a million times, and that we can handle
reflinking million different extents.  Add a couple of tests to ensure
that we can ^C and SIGKILL our way out of long-running reflinks.

v2: Don't run the signal tests on NFS, as we cannot interrupt NFS
clone operations.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
[hch@lst.de: don't run on NFS]
Signed-off-by: Christoph Hellwig <hch@lst.de>