ipv6: Optimize ipv6_change_dsfield().

Do not convert endian back and forth.
If the caller uses contant "mask" argument (and most callers do),
we can omit runtime endian conversion here.

Signed-off-by: YOSHIFUJI Hideaki <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/dsfield.h

···
 static inline void ipv6_change_dsfield(struct ipv6hdr *ipv6h,__u8 mask,
     __u8 value)
 {
-        __u16 tmp;
+	__be16 *p = (__force __be16 *)ipv6h;
 
-	tmp = ntohs(*(__be16 *) ipv6h);
-	tmp = (tmp & ((mask << 4) | 0xf00f)) | (value << 4);
-	*(__be16 *) ipv6h = htons(tmp);
+	*p = (*p & htons((((u16)mask << 4) | 0xf00f))) | htons((u16)value << 4);
 }