allow in-inode EAs on ext4 root inode

The ext3 root inode was treated specially with respect
to in-inode extended attributes, for reasons detailed
in the removed comment below.  The first mkfs-created
inodes would not get extra_i_size or the EXT3_STATE_XATTR
flag set in ext3_read_inode, which disallowed reading or
setting in-inode EAs on the root.

However, in ext4, ext4_mark_inode_dirty calls
ext4_expand_extra_isize for all inodes; once this is done
EAs may be placed in the root ext4 inode body.

But for reasons above, it won't be found after a reboot.

testcase:

setfattr -n user.name -v value mntpt/
setfattr -n user.name2 -v value2 mntpt/
umount mntpt/; remount mntpt/
getfattr -d mntpt/

name2/value2 has gone missing; debugfs shows it in the
inode body, but it is not found there by getattr.

The following fixes it up; newer mkfs appears to properly
zero the inodes, so this workaround isn't needed for ext4.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>