powerpc/boot: Fix missing check of lseek() return value

This is detected by Coverity scan: CID: 1440481
Signed-off-by: Bo YU <tsu.yubo@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/boot/addnote.c

@@ -223,7 +223,11 @@ main(int ac, char **av)
 	PUT_16(E_PHNUM, np + 2);
 
 	/* write back */
-	lseek(fd, (long) 0, SEEK_SET);
+	i = lseek(fd, (long) 0, SEEK_SET);
+	if (i < 0) {
+		perror("lseek");
+		exit(1);
+	}
 	i = write(fd, buf, n);
 	if (i < 0) {
 		perror("write");