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>

common/rc

@@ -317,6 +317,16 @@ _scratch_mkfs_options()
     echo $SCRATCH_OPTIONS $MKFS_OPTIONS $* $SCRATCH_DEV
 }
 
+_scratch_metadump()
+{
+	dumpfile=$1
+	options=
+
+	[ "$USE_EXTERNAL" = yes -a ! -z "$SCRATCH_LOGDEV" ] && \
+		options="-l $SCRATCH_LOGDEV"
+
+	xfs_metadump $options $SCRATCH_DEV $dumpfile
+}
 
 _setup_large_xfs_fs()
 {