ext4: only allow test_dummy_encryption when supported

ext4: only allow test_dummy_encryption when supported

Make the test_dummy_encryption mount option require that the encrypt
feature flag be already enabled on the filesystem, rather than
automatically enabling it.  Practically, this means that "-O encrypt"
will need to be included in MKFS_OPTIONS when running xfstests with the
test_dummy_encryption mount option.  (ext4/053 also needs an update.)

Moreover, as long as the preconditions for test_dummy_encryption are
being tightened anyway, take the opportunity to start rejecting it when
!CONFIG_FS_ENCRYPTION rather than ignoring it.

The motivation for requiring the encrypt feature flag is that:

- Having the filesystem auto-enable feature flags is problematic, as it
  bypasses the usual sanity checks.  The specific issue which came up
  recently is that in kernel versions where ext4 supports casefold but
  not encrypt+casefold (v5.1 through v5.10), the kernel will happily add
  the encrypt flag to a filesystem that has the casefold flag, making it
  unmountable -- but only for subsequent mounts, not the initial one.
  This confused the casefold support detection in xfstests, causing
  generic/556 to fail rather than be skipped.

- The xfstests-bld test runners (kvm-xfstests et al.) already use the
  required mkfs flag, so they will not be affected by this change.  Only
  users of test_dummy_encryption alone will be affected.  But, this
  option has always been for testing only, so it should be fine to
  require that the few users of this option update their test scripts.

- f2fs already requires it (for its equivalent feature flag).

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Link: https://lore.kernel.org/r/20220519204437.61645-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>