Standardise and add safety comments.

These will soon be required by a lint.

Bug: 290018030
Test: m rust
Change-Id: If62281a5eb2160061a11f28588b7a14e32ce265b
diff --git a/encryptedstore/src/main.rs b/encryptedstore/src/main.rs
index 86fa6da..1a16f49 100644
--- a/encryptedstore/src/main.rs
+++ b/encryptedstore/src/main.rs
@@ -153,6 +153,9 @@
     let mountpoint = CString::new(mountpoint.as_os_str().as_bytes())?;
     let fstype = CString::new("ext4").unwrap();
 
+    // SAFETY: The source, target and filesystemtype are valid C strings. For ext4, data is expected
+    // to be a C string as well, which it is. None of these pointers are retained after mount
+    // returns.
     let ret = unsafe {
         libc::mount(
             source.as_ptr(),