Fix a warning that has been annoying me for some time:

"kern/sys_generic.c:358: warning: cast discards qualifiers from pointer
   target type"
The idea for using the uintptr_t intermediate cast for de-constifying
a pointer was hinted at by bde some time ago.

sys/kern/sys_generic.c

@@ -355,7 +355,7 @@ dofilewrite(p, fp, fd, buf, nbyte, offset, flags)
 	int didktr = 0;
 #endif
 
-	aiov.iov_base = (void *)buf;
+	aiov.iov_base = (void *)(uintptr_t)buf;
 	aiov.iov_len = nbyte;
 	auio.uio_iov = &aiov;
 	auio.uio_iovcnt = 1;