ext4: do not advertise encryption support when disabled

The sysfs file /sys/fs/ext4/features/encryption was present on kernels
compiled with CONFIG_EXT4_FS_ENCRYPTION=n.  This was misleading because
such kernels do not actually support ext4 encryption.  Therefore, only
provide this file on kernels compiled with CONFIG_EXT4_FS_ENCRYPTION=y.

Note: since the ext4 feature files are all hardcoded to have a contents
of "supported", it really is the presence or absence of the file that is
significant, not the contents (and this change reflects that).

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