vboxsf: Make vboxsf_dir_create() return the handle for the created file

Make vboxsf_dir_create() optionally return the vboxsf-handle for
the created file. This is a preparation patch for adding atomic_open
support.

Fixes: 0fd16957

 ("fs: Add VirtualBox guest shared folder (vboxsf) support")
Signed-off-by: Hans de Goede <hdegoede@redhat.com>