The extent_from_logical() helper looks up an extent item by a key,
allowing to do an inexact search when key->offset is -1.  It's never
expected to find such item, as it would break the allowed range of a
extent item offset.

The same error is already handled in btrfs_backref_iter_start() so add a
comment for consistency.
Signed-off-by: David Sterba <dsterba@suse.com>