From: Duan Jiong <redacted>
Date: Thu, 27 Feb 2014 17:14:41 +0800

If the neigh table's entries is less than gc_thresh1, the function
will return directly, and the reachabletime will not be recompute,
so the reachabletime can be guessed.

Signed-off-by: Duan Jiong <redacted>

It looks like this bug was unintentionally introduced by:

commit 2724680bceee94eac391552863771af105a7355c
Author: YOSHIFUJI Hideaki / 吉藤英明 [off-list ref]
Date:   Tue Jan 22 05:20:05 2013 +0000

    neigh: Keep neighbour cache entries if number of them is small enough.

Applied and queued up for -stable, thanks!