virtiofsd: Add support for extended setxattr

Add the bits to enable support for setxattr_ext if fuse offers it. Do not
enable it by default yet. Let passthrough_ll opt-in. Enabling it by deafult
kind of automatically means that you are taking responsibility of clearing
SGID if ACL is set.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Message-Id: <20210622150852.1507204-4-vgoyal@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  Fixed up double def in fuse_common.h