fusefs: don't send the namespace during listextattr

The FUSE_LISTXATTR operation always returns the full list of a file's
extended attributes, in all namespaces. There's no way to filter the list
server-side. However, currently FreeBSD's fusefs driver sends a namespace
string with the FUSE_LISTXATTR request. That behavior was probably copied
from fuse_vnop_getextattr, which has an attribute name argument. It's
been there ever since extended attribute support was added in r324620. This
commit removes it.

Reviewed by: cem
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21280