Retry flock() 3 times with 100ms delay inbetween

It seems with the recent debian updates on my machine, some change
of timing/scheduling has come which makes flock() fail when the
lock holder is being killed by the timeout code in forkrun()

For future reference: logs/20231026_apt_history.txt

src/fellow_log.c

@@ -5739,13 +5739,20 @@ fellow_log_init(const char *path, size_t wantsize, size_t objsz_hint,
 		    path, strerror(errno), errno);
 		goto err_free;
 	}
-	if (flock(ffd->fd, LOCK_EX | LOCK_NB)) {
+#define FLOCK_TRIES 3
+	for (i = 0; i < FLOCK_TRIES; i++) {
+		if (! flock(ffd->fd, LOCK_EX | LOCK_NB))
+			break;
 		assert(errno == EWOULDBLOCK);
-		ffd->diag("fellow: flock(%s) failed: %s (%d)"
-		    " - a fellow file can only be used once\n",
-		    path, strerror(errno), errno);
+		ffd->diag("fellow: %d/%d flock(%s) failed: %s (%d)\n",
+		    i + 1, FLOCK_TRIES, path, strerror(errno), errno);
+		(void) usleep(100 * 1000);	// 100ms
+	}
+	if (i == FLOCK_TRIES) {
+		ffd->diag("a fellow file can only be used once\n");
 		goto err_free;
 	}
+#undef FLOCK_TRIES
 	ffd->cap = FFD_CAN_ANY;
 
 	memset(hdr, 0, sizeof hdr);