rust: alloc: implement `ReallocFunc`

rust: alloc: implement `ReallocFunc`

`ReallocFunc` is an abstraction for the kernel's realloc derivates, such
as `krealloc`, `vrealloc` and `kvrealloc`.

All of the named functions share the same function signature and
implement the same semantics. The `ReallocFunc` abstractions provides a
generalized wrapper around those, to trivialize the implementation of
`Kmalloc`, `Vmalloc` and `KVmalloc` in subsequent patches.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241004154149.93856-5-dakr@kernel.org
[ Added temporary `allow(dead_code)` for `dangling_from_layout` to clean
  warning in `rusttest` target as discussed in the list (but it is
  needed earlier, i.e. in this patch already). Added colon. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>