ext4: add debugging /proc file showing file system options

Now that /proc/mounts is consistently showing only those mount options
which need to be specified in /etc/fstab or on the mount command line,
it is useful to have file which shows exactly which file system
options are enabled.  This can be useful when debugging a user
problem.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>