generic: test reads racing with slow reflink operations

generic: test reads racing with slow reflink operations

XFS has a rather slow reflink operation.  While a reflink operation is
running, other programs cannot read the contents of the source file,
which is causing latency spikes.  Catherine Hoang wrote a patch to
permit reads, since the source file contents do not change.  This is a
functionality test for that patch.

[zlang@ adds `_supported_fs generic`]

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Catherine Hoang <catherine.hoang@oracle.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>