rust/revocable: add try_access_with() convenience method

Revocable::try_access() returns a guard through which the wrapped object
can be accessed. Code that can sleep is not allowed while the guard is
held; thus, it is common for the caller to explicitly drop it before
running sleepable code, e.g:

    let b = bar.try_access()?;
    let reg = b.readl(...);

    // Don't forget this or things could go wrong!
    drop(b);

    something_that_might_sleep();

    let b = bar.try_access()?;
    let reg2 = b.readl(...);

This is arguably error-prone. try_access_with() provides an arguably
safer alternative, by taking a closure that is run while the guard is
held, and by dropping the guard automatically after the closure
completes. This way, code can be organized more clearly around the
critical sections and the risk of forgetting to release the guard when
needed is considerably reduced:

    let reg = bar.try_access_with(|b| b.readl(...))?;

    something_that_might_sleep();

    let reg2 = bar.try_access_with(|b| b.readl(...))?;

The closure can return nothing, or any value including a Result which is
then wrapped inside the Option returned by try_access_with. Error
management is driver-specific, so users are encouraged to create their
own macros that map and flatten the returned values to something
appropriate for the code they are working on.

Suggested-by: Danilo Krummrich <[email protected]>
Reviewed-by: Benno Lossin <[email protected]>
Signed-off-by: Alexandre Courbot <[email protected]>
Reviewed-by: Joel Fernandes <[email protected]>
Acked-by: Miguel Ojeda <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Link `None`, `Some`, `Option` in doc-comment. - Danilo ]
Signed-off-by: Danilo Krummrich <[email protected]>

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