tune2fs: prevent stable_inodes feature from being cleared

tune2fs: prevent stable_inodes feature from being cleared

Similar to encrypt and verity, once the stable_inodes feature has been
enabled there may be files anywhere on the filesystem that require this
feature.  Therefore, in general it's unsafe to allow clearing it.  Don't
allow tune2fs to do so.  Like encrypt and verity, it can still be
cleared with debugfs if someone really knows what they're doing.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>