Return EOPNOTSUPP instead of ENOSYS for *xattr* syscalls

In current code, we're sending ENOSYS to target when a syscall for the
xattrs is done. This makes applications like ls complain loudly about
that and breaks scripts parsing the output. Moreover, iirc, implemented
features of filesystems are are sending EOPNOTSUPP (I've not checked so
I may be a little bit wrong on that...).
So, I'm proposing to return -EOPNOTSUPP and make ls happy

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
