vboxsf: don't allow to change the inode type

vboxsf_init_inode() is used both for initial setup of inode and for metadata
updates.  Tell it whether we are updating a live inode or setting up a new
instance and have it refuse to change type in the former case.

[fixed the braino caught by Hans de Goede <hdegoede@redhat.com>]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>