6lowpan: fix tag variable size

Function lowpan_alloc_new_frame() takes u8 tag as an argument. However,
its only caller, lowpan_process_data() passes down a u16. Hence,
the tag value can get corrupted. This prevent 6lowpan fragment reassembly of a
message when the fragment tag value is over 256.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Cc: Tony Cheneau <tony.cheneau@amnesiak.org>
Signed-off-by: David S. Miller <davem@davemloft.net>