rm -rf can fail sometimes with an error from fts_read. Make it honor

fflag to ignore fts_read errors, but stop deleting from that directory
because no further progress can be made.

When building a kernel with a high -j value on a high core count
machine, during the cleanobj phase we can wind up doing multiple rm
-rf at the same time for modules that have subdirectories. This
exposed this race (sometimes) as fts_read can return an error if the
directory is removed by another rm -rf. Since the intent of the -f
flag was to ignore errors, even if this was a bug in fts_read, we
should ignore the error like we've been instructed to do.

bin/rm/rm.c

@@ -335,7 +335,7 @@ err:
 		warn("%s", p->fts_path);
 		eval = 1;
 	}
-	if (errno)
+	if (!fflag && errno)
 		err(1, "fts_read");
 	fts_close(fts);
 }