Make FilePermissions constructible from mode_t without requiring cast to u32

I didn't find anywhere that the previous impl is being used.

This should fix the following error on platforms where libc::mode_t != u32,
such as aarch64-apple-darwin (u16).

    error[E0308]: mismatched types
      --> library/std/src/sys/pal/unix/fs/tests.rs:61:64
       |
    61 |         assert_eq!(format!("{:?}", FilePermissions::from_inner(libc::S_IFREG | mode)), expected);
       |                                    --------------------------- ^^^^^^^^^^^^^^^^^^^^ expected `u32`, found `u16`
       |                                    arguments to this function are incorrect
       |
    note: associated function defined here
      --> library/std/src/sys_common/mod.rs:80:8
       |
    80 |     fn from_inner(inner: Inner) -> Self;
       |        ^^^^^^^^^^
    help: you can convert a `u16` to a `u32`
       |
    61 |         assert_eq!(format!("{:?}", FilePermissions::from_inner((libc::S_IFREG | mode).into())), expected);

Author: dtolnay

library/std/src/sys/pal/unix/fs.rs

@@ -685,9 +685,9 @@ impl fmt::Debug for FileType {
     }
 }
 
-impl FromInner<u32> for FilePermissions {
-    fn from_inner(mode: u32) -> FilePermissions {
-        FilePermissions { mode: mode as mode_t }
+impl FromInner<mode_t> for FilePermissions {
+    fn from_inner(mode: mode_t) -> FilePermissions {
+        FilePermissions { mode }
     }
 }