rust: retain pointer mut-ness in `container_of!`

Avoid casting the input pointer to `*const _`, allowing the output
pointer to be `*mut` if the input is `*mut`. This allows a number of
`*const` to `*mut` conversions to be removed at the cost of slightly
worse ergonomics when the macro is used with a reference rather than a
pointer; the only example of this was in the macro's own doctest.

Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Signed-off-by: Tamir Duberstein <[email protected]>
Reviewed-by: Andreas Hindborg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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