Use `spare_capacity_mut` in the implementation of `push` to reduce the
use of `unsafe`. Both methods were added in commit 2aac4cd7dae3 ("rust:
alloc: implement kernel `Vec` type").
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>