Btrfs: Check metadata redundancy on balance

When converting a filesystem via balance check that metadata mode
is at least as redundant as the data mode. For example give warning
when:
-dconvert=raid1 -mconvert=single

Signed-off-by: Sam Tygier <samtygier@yahoo.co.uk>
[ minor message reformatting ]
Signed-off-by: David Sterba <dsterba@suse.com>