FROMLIST: mm: rust: add mmput_async support

Adds an MmWithUserAsync type that uses mmput_async when dropped but is
otherwise identical to MmWithUser. This has to be done using a separate
type because the thing we are changing is the destructor.

Rust Binder needs this to avoid a certain deadlock. See commit
9a9ab0d9 ("binder: fix race between mmput() and do_exit()") for
details. It's also needed in the shrinker to avoid cleaning up the mm in
the shrinker's context.

Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> (for mm bits)
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Bug: 370906207
Link: https://lore.kernel.org/all/20250115-vma-v12-5-375099ae017a@google.com/


Change-Id: Ie77da6df513990996ebbb7c5311bca9d37a4f150
Signed-off-by: Alice Ryhl <aliceryhl@google.com>