ext4: reserve xattr index for the Hurd

The Hurd is using inode fields which restricts it from using more
advanced ext4 file system features, due to design choices made over a
decade ago.  By giving the Hurd an extended attribute index field we
allow it to move the translator and author fields out of the core
inode fields, and hopefully we can get rid of ugly hacks such as
EXT4_OS_HURD and EXT4_MOUNT2_HURD_COMPAT somday.

For more information please see:
      https://summerofcode.withgoogle.com/projects/#5869799859027968

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