ext4: provide separate operations for sysfs feature files

ext4: provide separate operations for sysfs feature files

Currently sysfs feature files uses ext4_attr_ops as the file operations
to show/store data. However the feature files is not supposed to contain
any data at all, the sole existence of the file means that the module
support the feature. Moreover, none of the sysfs feature attributes
actually register show/store functions so that would not be a problem.

However if a sysfs feature attribute register a show or store function
we might be in trouble because the kobject in this case is _not_ embedded
in the ext4_sb_info structure as ext4_attr_show/store expect.

So just to be safe, provide separate empty sysfs_ops to use in
ext4_feat_ktype. This might safe us from potential problems in the
future. As a bonus we can "store" something more descriptive than
nothing in the files, so let it contain "enabled" to make it clear that
the feature is really present in the module.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>