common/xfs: capture realtime devices during metadump/mdrestore

common/xfs: capture realtime devices during metadump/mdrestore

If xfs_metadump supports the -r switch to capture the contents of
realtime devices and there is a realtime device, add the option to the
command line to enable preservation.

Similarly, if the dump file could restore to an external scratch rtdev,
pass the -r option to mdrestore so that we can restore rtdev contents.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>