FROMGIT: rust: alloc: add Vec::clear

Our custom Vec type is missing the stdlib method `clear`, thus add it.
It will be used in the miscdevice sample.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250502-vec-methods-v5-1-06d20ad9366f@google.com


Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Bug: 414994413
(cherry picked from commit a1e4d5c9
 https://github.com/Rust-for-Linux/linux.git

 alloc-next)
Change-Id: If3e0171d6b2298807dbf7abc36f110a5457ff4f9
Signed-off-by: Alice Ryhl <aliceryhl@google.com>