refactor FileInode to avoid using its parent's TreeInode::Entry

Summary:
Refactor FileInode and FileData so that they no longer store the
TreeInode::Entry that refers to this file.  The TreeInode::Entry is owned by
the parent TreeInode and is not safe to access without holding the TreeInode
contents_ lock, which the FileInode code never did.  Additionally, once a
FileInode is unlocked the TreeInode::Entry is destroyed.  Previously the
FileInode code would retain its pointer to this now invalid data, and could
still dereference it.

In the future I think it might be worth refactoring the FileInode/FileData
split a bit more, but this at least addresses the issues around invalid
accesses to entry_.

Reviewed By: bolinfest

Differential Revision: D4688058

fbshipit-source-id: 17d5d0c4e756afc6e3c4bb279cb1bb5231f3134f

eden/fs/inodes/Dirstate.cpp

@@ -248,7 +248,9 @@ class IgnoreChecker {
  }
 
  auto data = ignoreInode->getOrLoadData();
- data->materializeForRead(O_RDONLY);
+ auto materializeFuture = data->ensureDataLoaded();
+ // TODO: Use a future callback rather than blocking here
+ materializeFuture.get();
  ignore->loadFile(data->readAll());
  }