ANDROID: rust_binder: check FDA len before allocating

Userspace can currently make Rust Binder attempt to allocate a really
large amount of memory with kmalloc, which triggers a WARN_ON inside the
allocator. To fix this, validate the length before reserving space.

Bug: 415211355
Change-Id: Icc223a7489d9aead227d70c6d8b3749cb4347f55
Signed-off-by: Alice Ryhl <aliceryhl@google.com>