rust: vec: impl Default for Vec with any allocator

rust: vec: impl Default for Vec with any allocator

The implementation of Default is restricted to only work with kmalloc
vectors for no good reason. This means I have to use

	mem::replace(&mut my_vec, KVVec::new())

in Rust Binder instead of `mem::take(&mut my_vec)`. Thus, expand the
impl of Default to work with any allocator including kvmalloc.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://lore.kernel.org/r/20250610-vec-default-v1-1-7bb2c97d75a0@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>