vop_fsync_debugprepost(): take account of nullfs

[markj]

I reproduced the problem using a -o nocache nullfs mount, since that disables writing with shared locks held (why?). This patch fixes the crash.

[kib]

In D51730#1181610, @markj wrote:

I reproduced the problem using a -o nocache nullfs mount, since that disables writing with shared locks held (why?).

Because with nocache we are guaranteed that new nullfs vnode must be instantiated with each bypass call. That requires the lower vnode to be locked exclusively.