fix clippy warning casting u64 to usize

[rtczza]

fix clippy warning casting u64 to usize may truncate the value on targets with 32-bit wide pointer

clippy tips:

warning: casting `u64` to `usize` may truncate the value on targets with 32-bit wide pointers
   --> src/firecracker/examples/uffd/uffd_utils.rs:185:35
    |
185 |         let backing_memory_size = file_meta.len() as usize;
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: if this is intentional allow the lint with `#[allow(clippy::cast_possible_truncation)]` ...
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_truncation
    = note: requested on the command line with `-W clippy::cast-possible-truncation`
help: ... or use `try_from` and handle the error accordingly
    |
185 |         let backing_memory_size = usize::try_from(file_meta.len());
    |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[rtczza]

What modifications should I make to pass the CI/CD checks?