rust: mm: mark VmaNew as transparent

Unsafe code in VmaNew's methods assumes that the type has the same layout
as the inner `bindings::vm_area_struct`.  This is not guaranteed by the
default struct representation in Rust, but requires specifying the
`transparent` representation.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: dcb81ae ("mm: rust: add VmaNew for f_ops->mmap()")
Signed-off-by: Baptiste Lepers <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Cc: Alex Gaynor <[email protected]>
Cc: Andreas Hindborg <[email protected]>
Cc: Björn Roy Baron <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Danilo Krummrich <[email protected]>
Cc: Gary Guo <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Cc: Trevor Gross <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: BLepers

rust/kernel/mm/virt.rs

@@ -209,6 +209,7 @@ impl VmaMixedMap {
 ///
 /// For the duration of 'a, the referenced vma must be undergoing initialization in an
 /// `f_ops->mmap()` hook.
+#[repr(transparent)]
 pub struct VmaNew {
     vma: VmaRef,
 }