smbfs_close(). This fixes paging to and from mmap()'d regions of smbfs
files after the descriptor has been closed, and makes thttpd, GNU ld,
and perhaps more things work that depend on being able to do this.
PR: 48291
not to the parent's smbnode, which may be freed during the lifetime
of the child if the mount is forcibly unmounted. umount -f should now
work properly (ie. not panic) on smbfs mounts.
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
