rust: alloc: add Vec::insert_within_capacity

rust: alloc: add Vec::insert_within_capacity

This adds a variant of Vec::insert that does not allocate memory. This
makes it safe to use this function while holding a spinlock. Rust Binder
uses it for the range allocator fast path.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250502-vec-methods-v5-7-06d20ad9366f@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>