PMFS: Set initial root inode permissions to drwxr-xr-x

For what it's worth, these are the default root inode permissions for
ext4 as well.  Previously our root inode permissions were drwxrwxrwt.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Acked-by: Rob Gittins <rob.gittins@intel.com>