relax an assert in zfsctl_snapdir_lookup to match r323578

Since r323578 we may remove the last reference to a covered vnode with
vrele() instead of vput(). So, v_usecount may be decremented before
the vnode is locked and zfsctl_snapdir_lookup may "catch" the vnode
with v_usecount of zero and v_holdcnt of one.

PR: 225795
Reported by: asomers
MFC after: 1 week