Unlock correct lock in ffs_snapblkfree().

It is possible for ffs_snapblkfree() to race and lock snaplock while
the devvp snapdata is instantiated, but no snapshots exist.  In this
case the loop over snapshots in ffs_snapblkfree() is not executed, and
the local variable vp is left initialized to NULL.

Unlock using &sn->sn_lock and not vp->v_vnlock.  For the inodes on the
snapshot list, the locks are same.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks

sys/ufs/ffs/ffs_snapshot.c

@@ -1935,7 +1935,7 @@ retry:
 	 */
 	if (error != 0 && wkhd != NULL)
 		softdep_freework(wkhd);
-	lockmgr(vp->v_vnlock, LK_RELEASE, NULL);
+	lockmgr(&sn->sn_lock, LK_RELEASE, NULL);
 	return (error);
 }