Add MAP_SHARED_VALIDATE & MAP_SYNC (#2499)

* Add MAP_SHARED_VALIDATE & MAP_SYNC

This pull adds two flags: MAP_SHARED_VALIDATE & MAP_SYNC implemented on
https://github.com/rust-lang/libc

The MAP_SHARED_VALIDATE flag was added to linux since 4.15, its a flag
that does two things: provide backwards compatibility with old mmap
implementations that don't check for flags, and allow new flags to be
added.

MAP_SYNC, on the other hand, was added to allow mmap to utilize Direct
Accses (DAX) on hardware that support it (non-volatile memory devices)
or in general: any ram-shaped filesystem.

both flags are available on both linux and android.

* fix CI

* fix CI

* add to changelog

* do not compile on hurd and uclibc

* do not compile on freebsdlike

* apply requested conditional compilation fixes

* apply requested conditional compilation fixes

changelog/2499.added.md

@@ -0,0 +1 @@
+`MAP_SHARED_VALIDATE` was added for all linux targets. & `MAP_SYNC` was added for linux with the exclusion of mips architecures, and uclibc

src/sys/mman.rs

@@ -41,6 +41,9 @@ libc_bitflags! {
         MAP_FILE;
         /// Share this mapping. Mutually exclusive with `MAP_PRIVATE`.
         MAP_SHARED;
+        /// Force mmap to check and fail on unknown flags. This also enables `MAP_SYNC`.
+        #[cfg(target_os = "linux")]
+        MAP_SHARED_VALIDATE;
         /// Create a private copy-on-write mapping. Mutually exclusive with `MAP_SHARED`.
         MAP_PRIVATE;
         /// Place the mapping at exactly the address specified in `addr`.
@@ -142,6 +145,10 @@ libc_bitflags! {
         /// Region grows down, like a stack.
         #[cfg(any(linux_android, freebsdlike, target_os = "openbsd"))]
         MAP_STACK;
+        /// Do not write through the page caches, write directly to the file. Used for Direct Access (DAX) enabled file systems.
+        // Available on Linux glibc and musl, MIPS* target excluded.
+        #[cfg(all(target_os = "linux", not(any(target_arch = "mips", target_arch = "mips64", target_arch = "mips32r6", target_arch = "mips64r6")), not(target_env = "uclibc")))]
+        MAP_SYNC;
         /// Pages in this mapping are not retained in the kernel's memory cache.
         #[cfg(apple_targets)]
         MAP_NOCACHE;