busy and we are making progress towards making them not busy. This is
needed because smbfs vnodes reference their parent directory but may
appear after their parent in the mount's vnode list; one pass over the
list is not sufficient in this case.
This stops attempts to unmount idle smbfs mounts failing with EBUSY.
file servers fail to do it in the right way.
New NFLUSHWIRE flag marks pending flush request(s).
NB: not all cases covered by this commit.
Obtained from: Darwin
