btrfs-progs: mkfs: check metadata redundancy

btrfs-progs: mkfs: check metadata redundancy

Currently BTRFS allows you to make bad choices of data and
metadata levels. For example -d raid1 -m raid0 means you can
only use half your total disk space, but will lose everything
if 1 disk fails. It should give a warning in these cases.

When making a filesystem, check that metadata mode is at least
as redundant as the data mode. For example give warning when:

	-d raid1 -m raid0

Signed-off-by: Sam Tygier <samtygier@yahoo.co.uk>
[make the check more visible in mkfs output]
Signed-off-by: David Sterba <dsterba@suse.cz>