mmap_xen: Update to bitflags 2.4.0

[vireshk]

Clippy gives this warning currently:

error: &-masking with zero
--> src/mmap_xen.rs:433:1
|
433 | / bitflags! {
434 | | /// Flags for the Xen mmap message.
435 | | pub struct MmapXenFlags: u32 {
436 | | /// Standard Unix memory mapping.
... |
446 | | }
447 | | }
| |_^
|
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bad_bit_mask
= note: #[deny(clippy::bad_bit_mask)] on by default
= note: this error originates in the macro __impl_bitflags which comes from the expansion of the macro bitflags (in Nightly builds, run with -Z macro-backtrace for more info)

Fix it.

Summary of the PR

Please summarize here why the changes in this PR are needed.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR are signed (with git commit -s), and the commit
  message has max 60 characters for the summary and max 75 characters for each
  description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[stefano-garzarella]

I don't know if it's a real bug or not, but if you want to keep the previous values, you can switch to bitflags 2.* and you shouldn't see the warning anymore.

[stefano-garzarella]

Not for this PR, but I was looking at the code, and I'm a bit suspicious of:

    /// Is standard Unix memory.
    pub fn is_unix(&self) -> bool {
        self.bits() == Self::UNIX.bits()
    }

Can a Unix memory have NO_ADVANCE_MAP flag set?
If it is the case is_unix() will return false, so we may need a mask there.

[vireshk]

Not for this PR, but I was looking at the code, and I'm a bit suspicious of:

    /// Is standard Unix memory.
    pub fn is_unix(&self) -> bool {
        self.bits() == Self::UNIX.bits()
    }

Can a Unix memory have NO_ADVANCE_MAP flag set? If it is the case is_unix() will return false, so we may need a mask there.

Unix can't have NO_ADVANCE_MAP flag. Only Grant mapping can have that.

[stefano-garzarella]

Unix can't have NO_ADVANCE_MAP flag. Only Grant mapping can have that.

Oh I see, I was confused by the negative flag!