ieee802154: reassembly: fix possible buffer overflow

The max_dsize attribute in ctl_table for lowpan_frags_ns_ctl_table is
configured with integer accessing methods. This patch change the
max_dsize attribute to int to avoid a possible buffer overflow.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>