- several variants of sysv fs are supported only r/o.  Driver does
   force r/o on mount, but doesn't do anything on remount.  As the
   result, one can remount them r/w and results are Not Pretty(tm).
   Missing checks added, code cleaned up. 

 - we had double-brelse() in v7fs - if sanity checks on root inode will
   succeed, but allocation of root dentry fails, we brelse() the same
   buffer_head twice.  Fixed.