ANDROID: rust: allow zero init for KABI members

Allow all-zero initialization for structures that have KABI padding
added. Otherwise, the new __kabi_reserved* fields would need to be
individually initialized. This fixes reported errors such as:

  error[E0063]: missing field `__kabi_reserved1` in initializer of `blk_mq_ops`
     --> /proc/self/cwd/common/rust/kernel/block/mq/operations.rs:216:42
      |
  216 |     const VTABLE: bindings::blk_mq_ops = bindings::blk_mq_ops {
      |                                          ^^^^^^^^^^^^^^^^^^^^ missing `__kabi_reserved1`

Bug: 151154716
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: If7996037e95b3947eb3b3afa9513cff18f4be116
Signed-off-by: Carlos Llamas <cmllamas@google.com>

rust/kernel/block/mq/gen_disk.rs

@@ -134,6 +134,8 @@ impl GenDiskBuilder {
             pr_ops: core::ptr::null_mut(),
             free_disk: None,
             poll_bio: None,
+            // SAFETY: Allow all zeros because of Android KABI members
+            ..unsafe { core::mem::MaybeUninit::zeroed().assume_init() }
         };
 
         // SAFETY: `gendisk` is a valid pointer as we initialized it above

rust/kernel/block/mq/operations.rs

@@ -237,6 +237,8 @@ impl<T: Operations> OperationsVTable<T> {
         map_queues: None,
         #[cfg(CONFIG_BLK_DEBUG_FS)]
         show_rq: None,
+        // SAFETY: Allow all zeros because of Android KABI members
+        ..unsafe { core::mem::MaybeUninit::zeroed().assume_init() }
     };
 
     pub(crate) const fn build() -> &'static bindings::blk_mq_ops {