Btrfs-progs: fsck: only allow partial opening under repair mode

Btrfs-progs: fsck: only allow partial opening under repair mode

The reason that we allow partial opening is that sometimes,
we may have some corrupted trees.(for example extent tree), for
fsck repair case, the broken tree may be rebuilt later.

So if users only want to do check but not repair anything, this
patch will make fsck return failure as soon as possible and
tell users that some critial roots have been corrupted.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>