xfs: support xfs_metadump with external logs

xfs: support xfs_metadump with external logs

When running xfstests with an external log, the metadump tests fail
with extra output like:

    +filesystem is marked as having an external log; specify logdev on the mount command line.
    +xfs_metadump: cannot read superblock for ag 0

Add a _scratch_metadump() function to handle different logdev
configurations automatically for metadump.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>