ffs_snapblkfree(): add a comment explaining lockmgr invocation

(cherry picked from commit 4559700)

sys/ufs/ffs/ffs_snapshot.c

@@ -1807,9 +1807,15 @@ ffs_snapblkfree(fs, devvp, bno, size, inum, vtype, wkhd)
 		VI_UNLOCK(devvp);
 		return (0);
 	}
+
+	/*
+	 * Use LK_SLEEPFAIL because sn might be freed under us while
+	 * both devvp interlock and snaplk are not owned.
+	 */
 	if (lockmgr(&sn->sn_lock, LK_INTERLOCK | LK_EXCLUSIVE | LK_SLEEPFAIL,
 	    VI_MTX(devvp)) != 0)
 		goto retry;
+
 	TAILQ_FOREACH(ip, &sn->sn_head, i_nextsnap) {
 		vp = ITOV(ip);
 		if (DOINGSOFTDEP(vp))