6lowpan: revert: add missing spin_lock_init()

Revert the commit 768f7c7c to initialize
spinlock in the more preferable way and make it static to avoid sparse
warning.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>