hostfs: Allow fsync on directories

Historically hostfs did not open directories on the host filesystem
for performance and memory reasons.
But it turned out that this optimization has a drawback.
Calling fsync() on a hostfs directory returns immediately
with -EINVAL as fsync is not implemented.
While this is behavior is strictly speaking correct common userspace
like dpkg(1) stumbles over that and makes it impossible to use
hostfs as root filesystem.
The fix is easy, wire up the existing host open/fsync functions
to the directory file operations.

Reported-by: Daniel Gröber <dxld@darkboxed.org>
Signed-off-by: Richard Weinberger <richard@nod.at>