Fix missing fields in MacOS build

[mkuchnik]

MacOS has additional file fields that cause the build to fail:

      error[E0063]: missing fields `crtime` and `flags` in initializer of `fuse3::path::reply::FileAttr`
         --> monarch_extension/src/readonly_fuse.rs:375:24
          |
      375 |             .unwrap_or(FileAttr {
          |                        ^^^^^^^^ missing `crtime` and `flags`

      error[E0063]: missing fields `crtime` and `flags` in initializer of `fuse3::path::reply::FileAttr`
         --> monarch_extension/src/readonly_fuse.rs:161:8
          |
      161 |     Ok(FileAttr {
          |        ^^^^^^^^ missing `crtime` and `flags`

      error[E0063]: missing fields `crtime` and `flags` in initializer of `fuse3::path::reply::FileAttr`
         --> monarch_extension/src/chunked_fuse.rs:173:8
          |
      173 |     Ok(FileAttr {
          |        ^^^^^^^^ missing `crtime` and `flags`

      For more information about this error, try `rustc --explain E0063`.

on v0.4.0 this is:

 error[E0063]: missing fields `crtime` and `flags` in initializer of
      `fuse3::path::reply::FileAttr`
        --> monarch_extension/src/chunked_fuse.rs:83:8
         |
      83 |     Ok(FileAttr {
         |        ^^^^^^^^ missing `crtime` and `flags`

      For more information about this error, try `rustc --explain E0063`.

The first commit was tested on top of v0.4.0 (and thus can be cherry-picked into the tagged v0.4.0). The second was necessary on top of main. All changes are guarded on MacOS builds so no behavior should change for other platforms.