Add fctrl::SealFlag::F_SEAL_FUTURE_WRITE (#2213)

* Add `fctrl::SealFlag::F_SEAL_FUTURE_WRITE`

Add `fctrl::SealFlag::F_SEAL_FUTURE_WRITE`, a file sealing flag
available since Linux 5.1 which prevents modification except via shared
writable mappings that were created prior to the seal being set.

* Add changelog entry

* Use `linux_android` alias

Co-authored-by: SteveLauC <[email protected]>

---------

Co-authored-by: SteveLauC <[email protected]>

Author: tdryer

changelog/2213.added.md

@@ -0,0 +1 @@
+Added `fctrl::SealFlag::F_SEAL_FUTURE_WRITE`

src/fcntl.rs

@@ -420,6 +420,10 @@ libc_bitflags!(
         F_SEAL_GROW;
         /// The file contents cannot be modified.
         F_SEAL_WRITE;
+        /// The file contents cannot be modified, except via shared writable mappings that were
+        /// created prior to the seal being set. Since Linux 5.1.
+        #[cfg(linux_android)]
+        F_SEAL_FUTURE_WRITE;
     }
 );