9P: fix return value in v9fs_fid_xattr_set

v9fs_fid_xattr_set is supposed to return 0 on success.

This corrects the behaviour introduced in commit
bdd5c28d
"9p: fix return value in case in v9fs_fid_xattr_set()"

(The function returns a negative error on error, as expected)

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>