vboxsf: Add vboxsf_[create|release]_sf_handle() helpers

Factor out the code to create / release a struct vboxsf_handle into
2 new helper functions.

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>