libext2fs: add gnu.translator support

libext2fs: add gnu.translator support

The support of setting (and reading) of passive translators from
GNU/Linux has been added to the Linux kernel by the commit [1].
The name index '10' has been reserved for GNU/Hurd.

Hurd passive translators are stored as a xattr value with name
"gnu.translator" [2].

If "gnu.translator" xattr value has been set before calling
mkfs.ext2, it will segfault since "gnu." is not present in
ea_names[].

$ setfattr -n gnu.translator -v "/hurd/exec\0" ${TARGET_DIR}/servers/exec
$ mkfs.ext2 -d ${TARGET_DIR} -o hurd -O ext_attr rootfs.ext2 "1G"

Adding "gnu." to ea_names[], allow to create ext2 filesystem
for GNU/Hurd with passive translator already set.

[1] https://git.savannah.gnu.org/cgit/hurd/hurd.git/commit/?id=a04c7bf83172faa7cb080fbe3b6c04a8415ca645
[2] https://lists.gnu.org/archive/html/bug-hurd/2016-08/msg00075.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>