gmirror: Remove a last-minute INVARIANTS breakage in r341840

I mistakenly added a lock assertion to this routine at the last minute
without confirming it was held during g_mirror_create. It isn't (it isn't
even initialized yet). Mea culpa. Access is exclusive in both callers,
just not always by that particular lock.

Reported by: lwhsu
X-MFC-With: r341840, r341674