Fix Linux's CMSG_NXTHDR and CMSG_SPACE definitions.

This is an error from PR rust-lang#1098.  The wrong definitions coincidentally
work on Linux/glibc, but fail on Linux/musl.

src/unix/notbsd/mod.rs

@@ -985,6 +985,10 @@ pub const ARPHRD_IEEE802154: u16 = 804;
 pub const ARPHRD_VOID: u16 = 0xFFFF;
 pub const ARPHRD_NONE: u16 = 0xFFFE;
 
+fn CMSG_ALIGN(len: usize) -> usize {
+    len + mem::size_of::<usize>() - 1 & !(mem::size_of::<usize>() - 1)
+}
+
 f! {
     pub fn CMSG_FIRSTHDR(mhdr: *const msghdr) -> *mut cmsghdr {
         if (*mhdr).msg_controllen as usize >= mem::size_of::<cmsghdr>() {
@@ -996,17 +1000,19 @@ f! {
 
     pub fn CMSG_NXTHDR(mhdr: *const msghdr,
                        cmsg: *const cmsghdr) -> *mut cmsghdr {
-        if cmsg.is_null() {
-            return CMSG_FIRSTHDR(mhdr);
+        if (*cmsg).cmsg_len < mem::size_of::<cmsghdr>() {
+            return 0 as *mut cmsghdr;
         };
-        let pad = mem::align_of::<cmsghdr>() - 1;
-        let next = cmsg as usize + (*cmsg).cmsg_len as usize + pad & !pad;
+        let next = (cmsg as usize + CMSG_ALIGN((*cmsg).cmsg_len as usize))
+            as *mut cmsghdr;
         let max = (*mhdr).msg_control as usize
             + (*mhdr).msg_controllen as usize;
-        if next < max {
-            next as *mut cmsghdr
-        } else {
+        if (next.offset(1)) as usize > max
+            || next as usize + CMSG_ALIGN((*next).cmsg_len) > max
+        {
             0 as *mut cmsghdr
+        } else {
+            next as *mut cmsghdr
         }
     }
 
@@ -1015,12 +1021,12 @@ f! {
     }
 
     pub fn CMSG_SPACE(length: ::c_uint) -> ::c_uint {
-        let pad = mem::align_of::<cmsghdr>() as ::c_uint - 1;
-        mem::size_of::<cmsghdr>() as ::c_uint + ((length + pad) & !pad)
+        (CMSG_ALIGN(length as usize) + CMSG_ALIGN(mem::size_of::<cmsghdr>()))
+            as ::c_uint
     }
 
     pub fn CMSG_LEN(length: ::c_uint) -> ::c_uint {
-        mem::size_of::<cmsghdr>() as ::c_uint + length
+        CMSG_ALIGN(mem::size_of::<cmsghdr>()) as ::c_uint + length
     }
 
     pub fn FD_CLR(fd: ::c_int, set: *mut fd_set) -> () {