rtsold: Use correct error in llflags_get()

Set errno to EINVAL if the name overflows the buffer, as was done
before commit ecce515d54bc.

Reviewed by: dab, markj
Fixes: ecce515d54bcea54fea03f731aad646c87761d22
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D34354