On ea_inode-enabled ext4 open_by_handle() (as well as knfsd, etc.) can get to EA inodes as long as it knows their inumbers - just pass it an fhandle with zeroed version bytes and the right inumber in it. AFAICS, it's Not Nice(tm), especially since you can write to those, whether they are shared or not. Should we make ext4_nfs_get_inode() check for EXT4_EA_INODE_FL and fail if it's set?
