ARM: net: bpf_jit: fix emit_swap16() for non ARMv6+.

The original code was generating an lsl instructions using the value
of ARM_R8 (skb_headlen, possibly uninitialized if no skb_headlen
access was required) as a shift amount.

Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Acked-by: Mircea Gherzan <mgherzan@gmail.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>