before the struct file is fully initialized in vn_open(), in particular,
fp->f_vnode is NULL. Other thread calling file operation before f_vnode
is set results in NULL pointer dereference in devvn_refthread().
Initialize f_vnode before calling d_fdopen() cdevsw method, that might
set file ops too.
Reported and tested by: Chris Timmons <cwt networks cwu edu>
(RELENG_7 version)
MFC after: 3 days
0e9bd89d7d