ext4: optimize Hurd tests when reading/writing inodes

Set a in-memory superblock flag to indicate whether the file system is
designed to support the Hurd.

Also, add a sanity check to make sure the 64-bit feature is not set
for Hurd file systems, since i_file_acl_high conflicts with a
Hurd-specific field.

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

diff --cc fs/ext4/ext4.h
Simple merge

diff --cc fs/ext4/inode.c
Simple merge

diff --cc fs/ext4/super.c
Simple merge