ext4: clean up superblock encryption mode fields

The superblock fields s_file_encryption_mode and s_dir_encryption_mode
are vestigal, so remove them as a cleanup.  While we're at it, allow
file systems with both encryption and inline_data enabled at the same
time to work correctly.  We can't have encrypted inodes with inline
data, but there's no reason to prohibit unencrypted inodes from using
the inline data feature.

Change-Id: Ia90b7e24bcf9ebabef529b710d70bd8ba71a17a4
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: "Theodore Ts'o" <tytso@google.com>