nfsserver: use VOP_VPUT_PAIR().

Apply VOP_VPUT_PAIR() to the end of vnode operations after the
VOP_MKNOD(), VOP_MKDIR(), VOP_LINK(), VOP_SYMLINK(), VOP_CREATE().

Reviewed by: chs, mckusick
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation