vboxsf: Add support for the atomic_open directory-inode op

Opening a new file is done in 2 steps on regular filesystems:

1. Call the create inode-op on the parent-dir to create an inode
to hold the meta-data related to the file.
2. Call the open file-op to get a handle for the file.

vboxsf however does not really use disk-backed inodes because it
is based on passing through file-related system-calls through to
the hypervisor. So both steps translate to an open(2) call being
passed through to the hypervisor. With the handle returned by
the first call immediately being closed again.

Making 2 open calls for a single open(..., O_CREATE, ...) calls
has 2 problems:

a) It is not really efficient.
b) It actually breaks some apps.

An example of b) is doing a git clone inside a vboxsf mount.
When git clone tries to create a tempfile to store the pak
files which is downloading the following happens:

1. vboxsf_dir_mkfile() gets called with a mode of 0444 and succeeds.
2. vboxsf_file_open() gets called with file...