rust: uaccess: generalize userSliceReader to support any Vec

The UserSliceReader::read_all function is currently restricted to use
only Vec with the kmalloc allocator. However, there is no reason for
this limitation.

This patch generalizes the function to accept any Vec regardless of the
allocator used.

There's a use-case for a KVVec in Binder to avoid maximum sizes for a
certain array.

Link: https://github.com/Rust-for-Linux/linux/issues/1136


Signed-off-by: Filipe Xavier <felipeaggger@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250107-gen-userslice-readall-alloc-v2-1-d7fe4d19241a@gmail.com


[ Reflowed and slightly reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>