commit 083c73c2 upstream.

if 9P ->get_sb() fails late (at root inode or root dentry
allocation), we'll hit its ->kill_sb() with NULL ->s_root
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>