dmlogwrites: skip generic tests when external logdev in use

dmlogwrites: skip generic tests when external logdev in use

Currently, dm-logwrites and common/dmlogwrites don't seem to have any
means to coordinate the event numbers across multiple devices, and the
fstests setup code is sufficiently intense that it doesn't seem like
there's support for multi-disk filesystems.  For now, we'll _notrun the
tests when we have external log devices, even though that seems like
something we'd _really_ want to test.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>