btrfs: tree-checker: check for overlapping extent items

We're seeing a weird problem in production where we have overlapping
extent items in the extent tree.  It's unclear where these are coming
from, and in debugging we realized there's no check in the tree checker
for this sort of problem.  Add a check to the tree-checker to make sure
that the extents do not overlap each other.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>