mac80211: Only process mesh PREPs with equal seq number if metric is better.

This fixes routing loops in PREP propagation and is in accordance with Draft
11, Section: 11C.9.8.4.

Signed-off-by: Fabrice Deyber <fabricedeyber@agilemesh.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>