try to get rid of races in hostfs open()

In case of mode mismatch, do *not* blindly close the descriptor
another openers might be using right now.  Open the underlying
file with currently sufficient mode, then
	* if current mode has grown so that it's sufficient for
us now, just close our new fd
	* if current mode has grown and our fd is *not* enough
to cover it, close and repeat.
	* otherwise, install our fd if the file hadn't been
opened at all or dup2() our fd over the current one (and close
our fd).
Critical section is protected by mutex; yes, system-wide.  All
we do under it is a bunch of comparison and maybe an overwriting
dup2() on host.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/hostfs/hostfs_user.c

@@ -160,6 +160,11 @@ int fsync_file(int fd, int datasync)
 	return 0;
 }
 
+int replace_file(int oldfd, int fd)
+{
+	return dup2(oldfd, fd);
+}
+
 void close_file(void *stream)
 {
 	close(*((int *) stream));