btrfs-progs: volume: Fix a bug causing btrfs-find-root to skip first chunk

btrfs-progs: volume: Fix a bug causing btrfs-find-root to skip first chunk

There is a small bug from 2011, where btrfs_next_bg (formally
btrfs_next_metadata) function will always skip the first chunk.

That's OK for that time, as there is always 3 empty temporary chunks.
But now, we may ended up with only one metadata or system chunk, with
empty chunk auto-remove from kernel or new mkfs.btrfs.

So fix it by checking the initial value so btrfs_next_bg() will return
the first chunk if its *logical parameter is 0.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>