6lowpan: reassembly: un-export local functions

most of these are only used locally, make them static.
fold lowpan_expire_frag_queue into its caller, its small enough.

Cc: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>