Revert "netlink: Fix overallocation of netlink message buffers"

This patch was based on an incorrect assumption that the linear buffer
chain for an snl_writer only contained the netlink message body.

This reverts commit 828df4d36d.

Sponsored by:	AFRL, DARPA

sys/netlink/netlink_snl.h

@@ -1082,7 +1082,6 @@ snl_init_writer(struct snl_state *ss, struct snl_writer *nw)
 static inline bool
 snl_realloc_msg_buffer(struct snl_writer *nw, size_t sz)
 {
-	void *new_base;
 	uint32_t new_size = nw->size * 2;
 
 	while (new_size < nw->size + sz)
@@ -1091,27 +1090,23 @@ snl_realloc_msg_buffer(struct snl_writer *nw, size_t sz)
 	if (nw->error)
 		return (false);
 
-	new_base = snl_allocz(nw->ss, new_size);
-	if (new_base == NULL) {
+	if (snl_allocz(nw->ss, new_size) == NULL) {
 		nw->error = true;
 		return (false);
 	}
+	nw->size = new_size;
 
-	if (new_base == nw->ss->lb->base) {
-		/* Claim the entire linear buffer. */
-		nw->size = nw->ss->lb->size;
-		nw->ss->lb->offset = nw->ss->lb->size;
-	} else
-		nw->size = new_size;
+	void *new_base = nw->ss->lb->base;
+	if (new_base != nw->base) {
+		memcpy(new_base, nw->base, nw->offset);
+		if (nw->hdr != NULL) {
+			int hdr_off = (char *)(nw->hdr) - nw->base;
 
-	memcpy(new_base, nw->base, nw->offset);
-	if (nw->hdr != NULL) {
-		int hdr_off = (char *)(nw->hdr) - nw->base;
-
-		nw->hdr = (struct nlmsghdr *)
-		    (void *)((char *)new_base + hdr_off);
+			nw->hdr = (struct nlmsghdr *)
+			    (void *)((char *)new_base + hdr_off);
+		}
+		nw->base = (char *)new_base;
 	}
-	nw->base = (char *)new_base;
 
 	return (true);
 }