btrfs: simplify leaf traversal after path release in btrfs_next_old_leaf()

btrfs: simplify leaf traversal after path release in btrfs_next_old_leaf()

After releasing the path in btrfs_next_old_leaf(), we need to re-check
the leaf because a balance operation may have added items or removed the
last item. The original code handled this with two separate conditional
blocks, the second marked with a lengthy comment explaining a "missed
case".

Merge these two blocks into a single logical structure that handles both
scenarios more clearly.

Also update the comment to be more concise and accurate, incorporating the
explanation directly into the main block rather than a separate annotation.

Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>