Fix sync_all on macos/ios

[davazp]

sync_all should flush all metadata in macos/ios, so it should call fcntl with the F_FULLFSYNC flag as sync_data does.

Note that without this sync_data performs more flushes than sync_all on macos/ios.

[rust-highfive]

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[KodrAus]

Thanks for the PR @davazp

While the standard library docs don't make any explicit guarantees about the behaviour of sync_all beyond ensuring data reaches the filesystem (not necessarily the device) I would find it surprising to learn that sync_data had different durability guarantees than sync_all. At least there's a bit of an assumption that sync_data is a subset of sync_all, so this looks good to me!

@bors r+

[bors]

📌 Commit d602a6b has been approved by KodrAus

[bors]

💡 This pull request was already approved, no need to approve it again.

• There's another pull request that is currently being tested, blocking this pull request: upgrade rustdoc's pulldown-cmark to 0.4.1 #60140

[bors]

📌 Commit d602a6b has been approved by KodrAus

[bors]

⌛ Testing commit d602a6b with merge 3bee49f...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: KodrAus
Pushing 3bee49f to master...