Add mount_count file to hold the current MDS generation number. Note that

Add mount_count file to hold the current MDS generation number.  Note that
you cannot read this file from userspace, because it will (mistakenly) think
that this file lives on the OST and LBUG if that inode isn't allocated, or
you will get garbage data.  The kernel code has no problem reading it, of
course.  This LBUG will go away with EAs and/or when we start mounting the
ROOT directory, at which point you will not be able to read the file at all.