Reserve superblock fields s_lpf_ino and s_encryption_level

Reserve superblock fields s_lpf_ino and s_encryption_level

The s_lpf_ino field is intended to store the location of the lost and
found directory if the root directory becomes encrypted (which is not
yet supported).  The s_encryption_level field is designed to allow
support for future changes in the on-disk ext4 encryption format while
this feature under development, without having to burn a large number
of bits in the incompat feature flag.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>