DTYPE_VNODE.
Only acquire locks for O_EXLOCK/O_SHLOCK if file type is still vnode,
since we allow for fcntl(2) to process with advisory locks for
DTYPE_VNODE only. Another reason is that all fo_close() routines need to
check and release locks otherwise.
For O_TRUNC, call fo_truncate() instead of truncating the vnode.
Discussed with: rwatson
MFC after: 2 week