Fix make dev compile error on Mac

[blacktear23]

Execute make dev on MacOS will got compile error message:

error[E0432]: unresolved import `std::os::linux`
   --> src/util/rocksdb/mod.rs:530:26
    |
530 |             use std::os::linux::fs::MetadataExt;
    |                          ^^^^^ Could not find `linux` in `os`

error[E0599]: no method named `st_nlink` found for type `std::fs::Metadata` in the current scope
   --> src/util/rocksdb/mod.rs:531:52
    |
531 |             assert_eq!(fs::metadata(path).unwrap().st_nlink(), nlink);
    |                                                    ^^^^^^^^
    |
    = help: items from traits can only be used if the trait is in scope
    = note: the following trait is implemented but not in scope, perhaps add a `use` for it:
            candidate #1: `use std::os::macos::fs::MetadataExt;`


error: aborting due to 2 previous errors

error: Could not compile `tikv`.

[sre-bot]

Hi contributor, thanks for your PR.

This patch needs to be approved by someone of admins. They should reply with "/ok-to-test" to accept this PR for running test automatically.

[zhangjinpeng87]

LGTM

[overvenus]

LGTM