FROMLIST: mm: rust: add abstraction for struct mm_struct

These abstractions allow you to reference a `struct mm_struct` using
both mmgrab and mmget refcounts. This is done using two Rust types:

* Mm - represents an mm_struct where you don't know anything about the
  value of mm_users.
* MmWithUser - represents an mm_struct where you know at compile time
  that mm_users is non-zero.

This allows us to encode in the type system whether a method requires
that mm_users is non-zero or not. For instance, you can always call
`mmget_not_zero` but you can only call `mmap_read_lock` when mm_users is
non-zero.

The struct is called Mm to keep consistency with the C side.

The ability to obtain `current->mm` is added later in this series.

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-v12-1-375099ae017a@google.com/


Change-Id: I16eb9f344e43a3d7d7611abf27ed7295610a2ac3
Signed-off-by: Alice Ryhl <aliceryhl@google.com>