The plain fuse_reply_attr() function does not allow setting
fuse_attr.flags, so add this new function that does.

Make fuse_reply_attr() a wrapper around it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200909184028.262297-5-mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>