Change struct attribute to avoid aligned operations mismatch

Previous __alignment(4) allowed compiler to assume that operations are
performed on aligned region. On ARM processor, this led to alignment fault
as shown below:
trapframe: 0xda9e5b10
FSR=00000001, FAR=a67b680e, spsr=60000113
r0 =00000000, r1 =00000068, r2 =0000007c, r3 =00000000
r4 =a67b6826, r5 =a67b680e, r6 =00000014, r7 =00000068
r8 =00000068, r9 =da9e5bd0, r10=00000011, r11=da9e5c10
r12=da9e5be0, ssp=da9e5b60, slr=a054f164, pc =a054f2cc
<...>
udp_input+0x264: ldmia r5, {r0-r3, r6}
udp_input+0x268: stmia r12, {r0-r3, r6}

This was due to instructions which do not support unaligned access,
whereas for __alignment(2) compiler replaced ldmia/stmia with some
logically equivalent memcpy operations.
In fact, the assumption that 'struct ip' is always 4-byte aligned
is definitely false, as we have no impact on data alignment of packet
stream received.

Another possible solution would be to explicitely perform memcpy()
on objects of 'struct ip' type, which, however, would suffer from
performance drop, and be merely a problem hiding.

Please, note that this has nothing to do with
ARM32_DISABLE_ALIGNMENT_FAULTS option, but is related strictly to
compiler behaviour.

Submitted by:  Wojciech Macek <wma@semihalf.com>
Reviewed by:   glebius, ian
Obtained from: Semihalf

sys/netinet/ip.h

@@ -67,7 +67,7 @@ struct ip {
 	u_char	ip_p;			/* protocol */
 	u_short	ip_sum;			/* checksum */
 	struct	in_addr ip_src,ip_dst;	/* source and dest address */
-} __packed __aligned(4);
+} __packed __aligned(2);
 
 #define	IP_MAXPACKET	65535		/* maximum packet size */