ext4: use a table-driven handler for mount options

By using a table-drive approach, we shave about 100 lines of code from
ext4, and make the code a bit more regular and factored out.  This
will also make it possible in a future patch to use this table for
displaying the mount options that were specified in /proc/mounts.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>